Abstract: The two problems should be solved in query expansion.One is the origin of the expanded terms and the other is to select appropriate expanded terms from the expansion source．Therefore,this paper proposed query expansion method,in which the high quality relevant documents set is firstly obtained based on xml search results clustering and ranking model and it is regarded as the expansion source,and then the local word co-occurrence model combing xml documents structure features is applied to select the expanded query．The experimental data have proved two sides．On the one hand,the proposed expansion source acquisition method has obtained more relevant documents and the source has higher quality than those of traditional pseudo relevant feedback．On the other hand,compared to original query and no structure method,the selected expanded terms based on local word co-occurrence with XML structural features are more relevant to user’s query intension and lead to good performance in retrieval.

Key words: XML query expansion,Expansion source,Word co-occurrence model,XML structural feature