Software testing is an important technique that used to assure the quality of Web application. Test case automated generation is a crucial aspect of Web Service testing. Test case automated generation for Web Service includes test date and operation sequence generation. To resolve the problem that operation sequence couldn’t be generated automatically, this paper presents a new method. Firstly, a directed graph which used to express the relationship among the operations of Web Service is constructed, and then an operation sequences generation algorithm base on the directed graph is proposed. The redundancy of operation sequences is checked in this algorithm in order to avoid redundant test cases generation. A case study shows that the method is feasible.