Web services are becoming the most promising technology for cloud computing. When a single web service fails to satisfy service requestor's multiple function demands, web services need to be configured together to form a service composition with cost and other constraints. It is hard to obtain an optimal service composition by the existing service composition methods because the analysis of its dynamic execution process is neglected. This paper uses Petri Net to model and analyze the dynamic execution processes of service compositions. The effect of composite services reliability on the composite service performance is taken into account. The method regards the cost averaged for one time of successful execution of a composite as its actual executing cost, and then selects the composite services with the aim of minimizing the composite service execution cost. The simulation result shows that the proposed method is superior to other methods in execution cost.