Interlanguage has gradually become an important theory in Second language Acquisition since it was initially proposed in late 1960 s and early 1970 s. This paper mainly introduces the research process of interlangugae theory for 40 years, focusing on the proposal, development, and characteristics of interlanguage. At the same time, the thesis provides some suggestions and inspiration for future researches, especially the researches of language teaching, in terms of the research method, research content and research level. Interlanguage has gradually become an important theory in Second language Acquisition since it was initially proposed in late 1960 s and early 1970 s. This paper mainly introduces the research process of interlangugae theory for 40 years, focusing on the proposal, development, and characteristics of interlanguage The At the same time, the therapist, some of the researches of language teaching, in terms of the research method, research content and research level.