Abstract: Prototype theory has been developed very fast in many research fields since its establishment. With the development of it in the field of linguistic study, prototype theory was applied in many branches and sub-branches of linguistics, and was well applied in SLA, especially in lexical acquisition of second language. This study mainly concentrates on clarifying that through manifesting the application of prototype-set theory in the actual processes of pronoun and conjunction analysis, combining prototype theory and set theory of math, the prototype-set theory can be effectively applied in the process of lexical study. It is a good approach to lexical acquisition of second language, and it can offer us a good method and research perspective, so prototype-set theory is important for the lexical acquisition of second language, and researches of prototype-set theory in lexical acquisition of second language will be a new researching field with a great future.