In this paper, we propose a method for multi-term query expansions based on WordNet. In our approach, Hypernym/Hyponymy and Synonym relations in WordNet is used as the basic expansion rules. Then we use WordNet Lexical Chains and WordNet semantic similarity to assign terms in the same query into different groups with respect to their semantic similarities. For each group, we expand the highest terms in the WordNet hierarchies with Hypernym and Synonym, the lowest terms with Hyponym and Synonym, and all other terms with only Synonym. Furthermore, we use collection related term semantic network to remove the low-frequency and unusual words in the expansions. And our experiment reveals that our solution for query expansion can improve the query performance dramatically.