The present invention relates to a query processing method for searching XML data by which performance of XML query processing can be improved by equivalence class and a path expression reduction algorithm. The query processing method of the present invention includes a step of checking duplication of nodes found while an input XML document is parsed to reconstruct the XML document, storing other nodes, from which duplicated nodes checked in the checking step are excluded, in the form of an additional tree structure, and when a user inputs a query for XML data search, converting a path expression in the input query into the shortest path expression while traversing the tree structure.