Depending on the randomness of traffic demand and supply analysis, the paper presents the concept of unblocked reliability and the evaluation methods for road component and network system. Taking the unblocked reliability of the road system as optimization objective, the improvement optimization model of urban transportation network is built. Through the component importance analysis and genetic algorithm, the solution can be found effectively. In the end of the paper, an example is given to illuminate the practicability and maneuverability of the suggested probability assessment index and optimum methodology for the urban transportation network.