Compared to lexical synonymy in general context, the synonyms generated in specific context are often more creative and indeterminate. The interpretation of lexical synonymy in particular context often results in the emergent properties of its synonyms. This feature cannot be explained by semantic associative account. Combined with the concept of ad hoc category in cognitive psychology, this paper explores the cognitive premise and mechanism of lexical synonymy using the framework of relevance theory. We propose that the construction of lexical synonymy is the result of the mutual adjustments of the encyclopedic information and contextual assumption. This paper argues that the emergent property of lexical synonymy is inferred by the interlocutors in specific context through pragmatic inferential mechanism.