An important branch of applied linguistics mainly studies the process and results of people learning a second language, with the aim of objectively describing and scientifically explaining language learners' language competence and communicative competence.

The early theory of second language acquisition is an accessory of teaching method, which exists to improve teaching quality. 1967, Larry Selinker first put forward the theory of interlanguage in his monograph Language Transfer, and the theory of second language acquisition has its own research field and began to become an independent discipline. At present, the research on second language acquisition involves three major fields, namely, interlanguage research, internal factors of learners and external factors of learners.

From 65438 to 0994, Rod Ellis, a professor of second language English teaching in the School of Education of Temple University in Philadelphia, USA, wrote his masterpiece A Study of Second Language Acquisition, which became a classic textbook in this field. This book is divided into seven parts. The first part outlines the conceptual framework of the whole book. The second part summarizes the main theories about the nature of learners' language, including learners' errors, development patterns, language variables and pragmatic features. The third part explains second language acquisition from external factors, mainly expounding the role of social factors and input/interaction. The fourth part explains second language acquisition from internal factors, including language transfer, cognitive interpretation and language universality. The fifth part shifts the focus of discussion from learning to learners, and discusses individual differences and learning strategies in second language acquisition. The sixth part is about English classroom teaching, and discusses the related theories of classroom interaction and formal teaching. The seventh part is the summary of the book, which critically analyzes the current research on second language acquisition from three angles: data analysis, theoretical construction and practical application. The book is systematic and detailed, and can be used as a teaching material for graduate students in this field and a reference book for second language acquisition researchers. This book was introduced and published by Shanghai Foreign Language Teaching Press in 2000.

At present, this field is booming in China, and all major foreign language colleges and comprehensive universities have master's programs in this major, and the number of academic papers and graduation papers published is increasing every year. Well-known domestic scholars in this field include Professor Wen from Nanjing University, Professor from Guangdong University of Foreign Studies and Professor Jiang Jingyi from South China University of Technology.