Vague language, as an intrinsic nature of human language, is an universal phenomenon in our daily life. Pragmatic vagueness,the actual use of vague language in communication,is often used to reach certain purposes.This paper will mainly discuss the nature, deep mechanism and functions of pragmatic vagueness from the linguistic adaptation theory. Finally,the author points out that vagueness expressions are produced as a result of adaption to the certain contexts in order to ensure smooth communication.