Linguistics is closely related to lexicography.For the past few years,lexicography has witnessed a breakthrough with the rise of cognitive linguistics.Based on the research findings of cognitive linguistics,The New Oxford English Dictionary has adopted a brand new way of definition-prototypic definition,which is a great innovation in lexicography.However,its significance may not be restricted to lexicography.It may give great inspiration and enlightenment to vocabulary teaching and learning in EFL.Prototypic definition will help English learners to construct their semantic network and improve their lexical inferential capability.This paper tries to analyze the significance of the dictionary to the construction of semantic network in English vocabulary acquisition.