The American linguist Selinker put forward the concept of interlanguage in 1969. 1972 put forward the interlanguage hypothesis in his famous paper Interlanguage, which is an attempt to explore the language system and acquisition rules of second language learners in the process of acquisition, and is of great significance in the history of second language acquisition research. Interlanguage theory was first put forward by Selinker and others. Interlanguage refers to a transitional language between the mother tongue and the target language constructed by second language learners, which is in the process of continuous development and change and gradually approaches the target language. Selinker's interlanguage theory emphasizes three aspects in second language learning: ① What kind of cognitive process is responsible for interlanguage construction? ② What is the essence of interlanguage knowledge system? ③ Why can't most second language learners fully acquire the language competence of the target language? This is the earliest definition of interlanguage. The separation of the second language learner system, a system between the mother tongue and the target language in structure (an independent language system for second/foreign language learners, between the mother tongue and the target language in structure).