Abstract: Reliability index can be used to evaluate the performance of the road network .Traditional reliability index of road network is derived on the base of link unit ,which is difficult in data acquisition leading to its plight of application . This paper firstly analyzes the deficiencies of the traditional reliability index .Then ,by constructing a reasonable single-input and single-out structure model of road network based on travel time ,it derives a performance function to describe the reliability of road network .Based on the mechanism of First-Time Beyond ,this paper constructs a reliability model under the premise of the Poisson assumption .Finally ,the application of the mode is demonstrated on the Vissim plat-form .