Query expansion(QE) has been proved to be one of effective methods for improving the performance of the information retrieval(IR) system.Therefore,a new fuzzy QE method based on synonymy thesaurus is proposed,and the synonymy thesaurus is built based on the famous lexical database WordNet.In the synonymy thesaurus,the similarity between the synonyms is,which is obtained by Tanimoto coefficient.By using this synonymy thesaurus,query expansion can be done well.Then the fuzzy QE method is introduced into the document information retrieval system together with the modified vector space model.The experimental results show that the developed information retrieval system has got more effective performance than before by using the fuzzy query expansion method.One feature of the proposed information retrieval model is that it can be treated as one of simple semantic models.Another feature is that the expansion degree is controllable based on different thresholds.