This paper provides an overview and analysis of the relationship between universal grammar and the learnability of second languages.At the beginning,brief introduction are presented to give a main conception on U.G,second language acquisition and Chomsky's theory interrelated.In the main part,theories as to the role of universal grammar and the extent of mother tongue influence are presented and discussed;at the same time.samples are used to illustrate how the second language acquisition is constrained by principles and parameters of universal grammar,At last,summarize is presented with the availability of U.G to the study of second language.