In order to solve the problem that the reliability evaluation method of traditional transmission system has a high sampling frequency and a big time consuming, this paper proposes an improved state space partitioning based on automatic optimization stratified cluster sampling method. Through reasonable division of the whole state space, the method selects the analytic method and Monte Carlo Sampling to state choice in two subspaces, and through the automatic optimization stratified cluster sampling method, the method improves the optimal allocation of heavy fault space sampling times of simulation method and reduces the sampling variance of state space partitioning. The IEEE reliability test system is used to test the proposed methodology. Simulation results show that the method is not only suitable for different sizes of system evaluation, but also improves the computational efficiency of traditional state space partition method.