Reliability is a key issue of the service-oriented architecture (SOA) that is widely employed in distributed systems such as e-commerce and e-government. Redundancy based technologies are usually employed for building reliable service composition on top of unreliable Web services. This paper proposes a strategy for modeling and analyzing fault tolerant service composition. The strategy consists of service selection mechanism, service synchronization mechanism and task exception mechanism. Petri nets are used to construct different components of service composition. Once the model is constructed, theories of Petri nets help prove the consistency of processing states and reliability of the strategy. The corresponding enforcement method for constructing fault-tolerant service composition is proposed. Experiments are conducted to demonstrate the applicability and effectiveness of the fault tolerant strategy.