A system reliability evaluation method for routing policy is disclosed. The single minimal path of the routing policy includes plural arcs between a start node and a terminal node in a flow network. The method includes the steps of providing a virtual network in a computer for simulating the flow network; inputting a transmission requirement, a budget restriction and a time restriction; distributing the transmission requirement in a first minimal path of the virtual network for getting a first feasible probability; if the first minimal path is inactive, distributing the transmission requirement in a second minimal path of the virtual network and getting a first inactive probability of the first minimal path; getting a second feasible probability of the second minimal path; and computing a system feasible probability of the virtual network by the first feasible probability, the first inactive probability and the second feasible probability, defining the system reliability.