In view of the problem that the high-order fault state of the system is difficult to be considered in the reliability assessment of the grid operation, and the traditional reliability assessment method can not meet the calculation time requirements of the operational reliability index, an analytical method combined with the Monte Carlo method is proposed. The basic idea of the improved hybrid method is: firstly, the N-1 fault of the system is processed by the analytical method, and the initial reliability index is calculated; then, the high-order fault of the system is replaced by the expected accident chain, and the Monte Carlo method is used to sample the cascading fault. In the initial step, the intermediate link is judged and identified, and the intermediate reliability index is calculated. Finally, the reliability index of the system is calculated and the system operation reliability evaluation is realized. Through the analysis of RTS-79 system and a power grid in Guizhou Province, and comparing with the traditional Monte Carlo method, the correctness and validity of the proposed method are verified.