Path expression is one of the core components of most XML query languages, and many evaluation methods for path expression queries are proposed recently. However, there are few researches on the issue of path expression optimization. In this paper, two kinds of path expression optimizing principles are proposed, named path shorten and path complementing, respectively. The path shorten principle reduces the querying cost by shortening the path expressions with the knowledge of XML schema. While the path complementing principle tends to substitute the user queries with the equivalent lower-cost path expressions. The experimental results show that these two techniques can work on most path expression queries and largely improve the efficiency of path expression query processing.