The main reason of low precision in information retrieval (IR) is that it is difficult for the users to submit a precise query expression for their query intensions. Furthermore, XML documents have characteristics not only in the content, but also in its structure. Therefore it is more difficult for users to submit precise query expressions. In order to solve this problem, this paper puts forward a new query expansion method based on relevance feedback. It can help users to construct a content and structure query expression which can satisfy users' intentions. This method includes two steps. The first step is to expand keywords for finding the weighted keyword which can represent the user's intentions. The second step is structural expansion based on the weighted keywords. Finally a full-edged content-structure query is formalized. Experimental results show that the method can obtain better retrieval results. The average precision of prec@10 and prec@20 is 30% higher than the original query.