Interlanguage,which functions as a relatively systematic transition from initial knowledge of a language to native proficiency during the process of language acquisition,is always regarded as an unstable set of language characteristics produced by learners. Its fossilization is a universal phenomenon in SLA and it derives from multiple factors. Through the research into interlanguage and its fossilization,we should hold a totally different view from traditional one on the nature and function of teacher’s training, teaching materials and the handling of the learners’ errors, etc. Interlanguage, which functions as a relatively systematic transition from initial knowledge of a language to native proficiency during the process of language acquisition, always always as an unstable set of language characteristics produced by learners. Its fossilization is a universal phenomenon in SLA and it derives from multiple factors. Through the research into interlanguage and its fossilization, we should hold a totally different view from traditional one on the nature and function of teacher’s training, teaching materials and the handling of the learners’ errors, etc.