To solve the problem of lower precision caused by traditional query expansion technology, a new query expansion technique based on semantic context was proposed. The semantic context is constructed by WordNet knowledge base and related feedback documents. Firstly, the query words senses are confirmed by disambiguation with WordNet lexical database. Secondly, the initial expansion words are obtained according to the WordNet semantic hierarchy structure. Finally, the weight of the expansion terms is determined according to the overall correlation of candidate expansion terms and all the query words. These words whose weight is higher than weight threshold will be chosen as the final query expansion word. The experimental results show that the proposed method obviously improves the retrieval precision while preserving higher recall.