【Abstract】Interlanguage is a common phenomenon in foreign language learning process.It refers to the language system which is formed in a certain period in learners’ second language or foreign language learning process.In the past，behaviorist learning theory and mentalist learning theory has dominated language learning for a long period of time，but both of them have some deficiencies.This paper is to make a critical analysis on the factors contributing to the formation of interlanguage，which reflects the practical importance of interlanguage to language teaching and learning.
　　【Key Words】Interlanguage； Universal grammar； Language transfer； Interaction
　　Introduction
　　Interlanguage is a common phenomenon in language learning.It refers to the knowledge system which is formed in a certain period in learners’ second language or foreign language learning process.It is a transitional language system，which is changing and adjusting constantly with the increase of learners’ second language knowledge.The formation of interlanguage is caused by many factors，mainly including linguistic factor，psychological factor and social factor.Interlanguage is the results of multiple effects of these factors.
　　1.Universal Grammar
　　The linguistic on the formation of interlanguage is related to Noam Chomsky’s Universal Grammar（UG）firstly.UG is “the system of principles，conditions，and rules that are elements or properties of all human languages…the essence of human language”（Chomsky，1976； Cook