The semantic WS specification OWL-S is used to describe the application logic of composite service process. There is a need for formal modeling and analyzing techniques for reliable Web services and their composition because of dynamic and complex service composition process. In order to reduce the complexity of model, we use Petri nets to model atomic processes and seven basic structures of the Web service composition. In this model, input, output and precondition are represented through different kinds of tokens, effect is represented by the change of the token number during firing the transition, the composition model can be described unambiguously and the composite process can be analyzed and verified conveniently. Then, a definition of reachable service graph is given and algorithms are proposed to construct the Petri nets and verify the mode. Finally, the composite service model is applied to a case.