Abstract: The paper illustrates the causes of language fuzziness from the aspect of cognitive psychology,categorization on objective things or phenomena,language and communicative activities.It holds that in human cognitive ability there are uncertainty and limitation which result in the vagueness of the categorization of objective things or phenomena,and vagueness exists unavoidably in the language referring to those categories.Language and objective things or phenomena are connected by cognitive activities,so when we have vague reaction to objective things or phenomena the corresponding linguistic symbols are surely vague.Moreover,language vagueness becomes the necessity of communication because of its unique pragmatic function.