Abstract: Since Halliday and Hassan Hassan put forward the theory of discourse cohesion and coherence in 1976, it has had a great response in academic circles, and linguists at home and abroad have paid more and more attention to the study of discourse coherence, which has aroused further research by scholars. This paper analyzes the coherence of discourse by using speech act theory, and puts forward the expressions of tangible coherence, intangible coherence and intangible coherence in different levels of speech act theory. [Paper Keywords] Discourse coherence; Speech act theory; Coherence of expression is the core of discourse analysis. Due to the complexity of coherence in discourse analysis, there are no systematic standards and methods for the interpretation of coherence by experts and scholars at present. Halliday & Hassan Halliday &; In their article Coherence in English, Hassan believes that coherence is naturally formed on the basis of cohesion, and cohesion is an indispensable condition for coherence. This theoretical view immediately aroused great repercussions in the academic circles, which led to further research by scholars, and some scholars also put forward critical opinions from different angles. It focuses on the role of cohesion in discourse coherence, arguing that it cannot guarantee discourse coherence, and cohesion is neither a necessary nor a sufficient condition for achieving discourse coherence. Brown & You Brown &; In Yule's view, people do not rely on the formal markers of the text when interpreting it, but on the assumption of coherence of the text. Cohesion can be regarded as a linguistic symbolic relationship between propositions and an illocutionary act, while coherence is an illocutionary act, which is determined by its pragmatic function. Many domestic scholars, such as Cheng Yumin, Guo-Wen Huang, Hu Zhuanglin, Zhu Yongsheng and Zhang Delu, have explained and supplemented this theory. To sum up, scholars' opinions on revision can be roughly divided into three points: first, cohesion is an important condition for discourse coherence, but linguistic cohesion does not necessarily lead to semantic coherence. Secondly, discourse coherence is the result of the interaction between discourse and contextual factors, which exist in pragmatic presupposition and reasoning in social and cultural context or situational context in the process of communication. Thirdly, discourse coherence is a psychological representation, which is the result of psychological interaction between two communicators in a certain cognitive context. At present, people have no clear understanding of the relationship between cohesion and coherence, and whether cohesion can be used to infer the coherence and coherence degree of a text. These aspects need further study. Discourse coherence should be multifaceted, not only at the semantic level, but also at the pragmatic level and the psychological interaction between the two parties. They can be analyzed and studied within the framework of speech act theory. 1. Speech act theory was first put forward by the British philosopher Austin, and was later revised and developed by Searle. Austin believes that the object of language research should not be words and sentences, but behaviors completed through words and sentences; Any speech act consists of three sub-acts, namely illocutionary act 1 event act, illocutionary act and perlocutionary act. The illocutionary act expresses literal meaning; The illocutionary act expresses the implication or the speaker's communicative intention, which is also called illocutionary force; Speech act refers to the influence or effect of the intention of an act once it is understood by the addressee. If the text is regarded as a whole speech act, we will find that the coherence of the three levels is different, that is, the coherence of speech acts is manifested as tangible signs such as grammar, vocabulary, logical relationship and theme-rheme structure, which is characterized by tangible dominance; ; The coherence of illocutionary acts is derived from the speaker's meaning according to the social and cultural context and situational context, and its characteristics are implicit; ; The coherence of illocutionary acts is mainly manifested in the psychological interaction between the speaker and the receiver, which is invisible. Although different levels have different forms and characteristics, they are an organic whole and a complete speech act. Austin abstracted the above three behaviors from a complete speech act in an abstract way, instead of dividing a complete speech act into three pieces like a watermelon, and then cutting one of them into three small pieces. Abstraction is actually looking at a thing at different levels or from different angles. It can be said that the abstract behavior is not a combination relationship, but an implication relationship. As Austin explained, pronunciation behavior does not necessarily have ideographic behavior, but ideographic behavior must include pronunciation behavior. Similarly, illocutionary acts do not necessarily have illocutionary acts, but illocutionary acts must contain illocutionary acts; The illocutionary act is not necessarily the illocutionary act, but the illocutionary act must include the illocutionary act and the illocutionary act. This connotation relationship is one-way and transitive. So, what is the relationship between the three levels of coherence? They are also a kind of implication relationship. The difference is that this connotation relationship is bidirectional or mutually conditional, that is, tangible coherence-intangible coherence-intangible coherence. Its theoretical model can be expressed as follows: the context of illocutionary act is tangible coherence such as grammar, vocabulary, logical relationship and theme-rheme structure; Social and cultural context/situational context of illocutionary act-implicit coherence; Psychological interaction between two parties in speech act communication--invisible coherence. Second, the performance of discourse coherence in discourse. Inferring coherence from cohesion requires determining the performance of coherence in the text. Some scholars simply define coherence as "unity" and think that it is the connection between the text and the context in which the text is produced. Some scholars believe that discourse coherence is manifested in two kinds of contact: contextual contact and contextual contact. I agree with the above point of view that discourse coherence needs to show the interrelation between the internal components of the discourse, thus showing the integrity of meaning, and at the same time showing the connection between the discourse and the context, so as to realize its communicative function in the context. According to the theory of systemic functional linguistics, a text has three interrelated meanings: conceptual meaning, interpersonal meaning and textual meaning. From the perspective of discourse coherence, the coherence of conceptual meaning of discourse is mainly manifested in the consistency of discourse theme, expressing the experience of the same theme and the same event; The coherence of interpersonal meaning in the text is manifested in the consistency of tone and attitude, achieving the same communicative purpose; The coherence of the textual meaning of a text is manifested in the coherence of the whole text and the consistency of genre. As far as the relationship between text and context is concerned, text coherence should be manifested as the consistency between the text and the events it represents. Language is an integral part of the whole communication process to achieve communication goals; Sometimes it plays a major role; Sometimes it just plays an auxiliary role. In this way, the construction of the text itself should be adapted to the context in two aspects: 1 How the text is related to other related factors in the communication event, so as to form an overall communication event with it, including situational context and cultural factors that determine language communication, including other symbolic system features that realize meaningful communication with language, such as related things in the scene, related people, body language, spatial language, and other communicative behaviors of communicators. 2. The dependence of the text on the context should be shown inside the text. In a text, the coherence of the text is embodied in the cohesion mechanism of the text. The consistency of discourse theme is mainly reflected in the density and length of cohesion chain composed of reference and lexical cohesion, and the process continuity of transitivity structure; The consistency of mood and attitude in a text is manifested in the density and length of cohesive chains composed of words and modality that express the meaning of attitude, the harmony between adjacent question-and-answer pairs, and the continuity between mood structure and intonation consistent with communication goals. The continuity of the whole text is manifested in the continuity of thematic structure, information structure and unstructured cohesion. The consistency between text and context is mainly achieved through the external cohesion mechanism in the text. Thirdly, the mode of discourse coherence 1. Tangible coherence of illocutionary acts Since textual coherence refers to the meaningful connection between different components in a text, this connection first exists between sentences, that is, the connection between propositions contained in different sentences. It not only refers to the logical relationship between two or more propositions in time and space, cause and effect, turning point, concession, addition, examples and so on. It also refers to the cohesive relationship between theme and rheme, which is generally expressed through language forms. Searle divides illocutionary acts into speech acts and propositional acts based on Austin's speech act theory. Speech act refers to the act of saying certain words, phrases or sentences; Propositional behavior refers to the behavior completed by making reference or judgment. Its cohesive devices are tangible, and coherence is achieved through a series of cohesive devices such as structure or non-structure in the text. Look at the example: ijohgavemaryagoldwatch yesterday. They will get married next month. Here, the coherence of the two sentences is largely achieved through referential relationship: in the first sentence, they have referential relationship with John and Mary in the first sentence. This sentence has a certain meaning, which consists of the meanings of the words that make up the sentence, and refers to the state of the knife on the table. Coherence of a text is achieved by cohesive devices. From this perspective, cohesion is a necessary factor for a complete and unified text. If "coherence" is compared to a building, then "cohesion" is its foundation. A building can't be built without foundation, and there is no "coherence" without "cohesion". Second, the performance of discourse coherence in discourse. Inferring coherence from cohesion requires determining the performance of coherence in the text. Some scholars simply define coherence as "unity" and think that it is the connection between the text and the context in which the text is produced. Some scholars believe that discourse coherence is manifested in two kinds of contact: contextual contact and contextual contact. I agree with the above point of view that discourse coherence needs to show the interrelation between the internal components of the discourse, thus showing the integrity of meaning, and at the same time showing the connection between the discourse and the context, so as to realize its communicative function in the context. According to the theory of systemic functional linguistics, a text has three interrelated meanings: conceptual meaning, interpersonal meaning and textual meaning. From the perspective of discourse coherence, the coherence of conceptual meaning of discourse is mainly manifested in the consistency of discourse theme, expressing the experience of the same theme and the same event; The coherence of interpersonal meaning in the text is manifested in the consistency of tone and attitude, achieving the same communicative purpose; The coherence of the textual meaning of a text is manifested in the coherence of the whole text and the consistency of genre. As far as the relationship between text and context is concerned, text coherence should be manifested as the consistency between the text and the events it represents. Language is an integral part of the whole communication process to achieve communication goals; Sometimes it plays a major role; Sometimes it just plays an auxiliary role. In this way, the construction of the text itself should be adapted to the context in two aspects: 1 How the text is related to other related factors in the communication event, so as to form an overall communication event with it, including situational context and cultural factors that determine language communication, including other symbolic system features that realize meaningful communication with language, such as related things in the scene, related people, body language, spatial language, and other communicative behaviors of communicators. 2. The dependence of the text on the context should be shown inside the text. In a text, the coherence of the text is embodied in the cohesion mechanism of the text. The consistency of discourse theme is mainly reflected in the density and length of cohesion chain composed of reference and lexical cohesion, and the process continuity of transitivity structure; The consistency of mood and attitude in a text is manifested in the density and length of cohesive chains composed of words and modality that express the meaning of attitude, the harmony between adjacent question-and-answer pairs, and the continuity between mood structure and intonation consistent with communication goals. The continuity of the whole text is manifested in the continuity of thematic structure, information structure and unstructured cohesion. The consistency between text and context is mainly achieved through the external cohesion mechanism in the text.