Disclosed is an extend stochastic Petri nets (ESPN)-based method for modeling and representing process reliability of a flexible manufacturing system (FMS), falling within the technical field of flexible manufacturing. Modular division is performed on an FMS, and performance parameters of various process modules are acquired; ESPN septuple models corresponding to subnets of Petri nets are constructed according to the performance parameters of the various process modules; based on a process flow, the subnets of Petri nets are combined according to a set control approach; and numerical values of transitions with arbitrary distributions in a complete ESPN model are solved through numerical simulation. In the disclosure, the accuracy for evaluating the process reliability of the FMS is ensured.