Abstract: Focusing on the query algorithm of XML data for path expressions and based on the marked suffix tree (MST), an optimizing algorithm is proposed for the common sub-queries in XML data for regular path expressions to improve the querying efficiency of path expressions through the redundancy eliminating technique. After a test done on the benchmark REOA, a performance analysis is made to the optimizing algorithm for the common sub-queries on MST basis. The result shows that the redundancy eliminating technique based on MST and used in XML data for path expressions will greatly improve the querying efficiency.