Metonymy is an important means for people to know the world and enrich the language; and it is a way of thinking used widely in people's daily life. This paper illustrates firstly the cognitive nature of metonymy in terms of its definition, classification and contiguity notion. Based on this, the author then studies the meaning extension and lexical conversion of vocabulary from the perspective of metonymy, and concludes that understanding the cognitive nature of metonymy can greatly promote the efficiency of vocabulary teaching and help to expand students' vocabulary amount.