In order to solve the problems of system interface and optimal service matching, this study puts forward a kind of Web service composition system which is based on the time-varying Petri net. It brings in the probability of service call to change the invalid trouble in distributed network. It uses the structure of Petri net to achieve sound dynamic balance in the composition success rate and user satisfaction. For demonstrating the correctness and validity of the system, this paper presents a kind of global fish algorithm based on backward theory to get the optimal combination of QoS. And it defines the evaluation index of system performance. By comparing different efficiency of combinations, it proves the feasibility of the composite method and can meet the needs of specific users of the service function.