The generated system state space can be very large when a complex software system is tested.In order to avoid the unnecessary traversing of the entire state space, a new method is presented based on the combination of CPN modeling and on-the-fly method to generate test cases.During such a process, only part of the state space is traversed according to the tester's personnel interest.Firstly, both the definitions of CPN and the extended reachability graph are introduced, and the related concepts relating to the on-the-fly testing method, including system specification, test purpose, synchronous product and test cases, are introduced.Secondly, a synchronous product algorithm is implemented, and the test cases are designed to test the algorithm as well.Finally, an implementation under test is selected to sample the combination method of CPN modeling and on-the-fly method.The interactions between the tester and the implementation under test are realized through an adapter, and the test cases are generated and executed simultaneously.Thus the feasibility and the effectiveness of the proposed method are verified.