The structural characteristics of XML database document have been fully made use of and combined with the encoding theory of Dewey code to design an encrypted XML algorithm called Index Layered Itersection Scan Algorithm(ILISA)under the model of Database as A Service(DAS).The data retrieval of tree structure was turned into the data retrieval of sequential list form.The authors applied the interpolation search algorithm instead of depth-first or breadth-first search,which had brought a good time complexity.And a data structure of XML index table was designed to reduce the search space greatly.Finally,the complexity of ILISA was described and the efficiency of ILISA was proved.