【Abstract】This paper reviews the emergence and development about “the critical period hypothesis” of second language acquisition（hereinafter called “the hypothesis”） . The author gives the corresponding suggestions according to the hypothesis.
　　【Key words】Second Language Acquisition; the Critical Period Hypothesis; target language
　　【作者簡介】白航凡（1995.01-），女，汉族，河北定州人，河北大学外国语学院，硕士研究生在读，研究方向：外国语言学及应用语言学专业。
　　Ⅰ. Introduction
　　The critical period hypothesis of language acquisition was proposed by Lenneberg （1967） ， which has been widely recognized in the field of first language acquisition. However， its controversy in the field of second language（hereinafter called L2） acquisition is far from over.
　　Ⅱ. The Critical Period Hypothesis of the first language acquisition
　　The Critical Period Hypothesis believes that there is a period that is crucial to language acquisition. But domestic and foreign scholars have different views on the specific time of the critical period. Lenneberg （1967） believed that the critical period of language learning was from the age of 2 to adolescence. Krashen （1973） believes that human’s peak period of language development is at 5 years old.
　　Ⅲ. The Critical Period Hypothesis of the second language acquisition
　　Some theoretical results in the field of the first language acquisition are often applied to the field of the L2 acquisition. Many linguists have used different experimental methods to prove their views.
　　The viewpoint of the proponents
　　The proponents mainly confirm the rationality of their views from three aspects： Firstly， the effect in learning L2 is more dominant in children than adults; secondly， the factor of age is important to affect L2 learning; Thirdly， a few of the adult L2 learners are able to reach the level of their own native speakers.
　　Through one year study， Snow