The reliability and operation strategy of battery energy storage system (BESS) are important to the reliability of power system. Based on analyzing the structure of two-stage grid-connected BESS, the reliability model of grid-connected BESS is established using the state space method. Then the reliability indices of BESS can be acquired. Reliability evaluation is made on a distribution system by using the software kits RAMSES, while allocating BESS in different conditions. Integrated analysis of the reliability of BESS accessing the distribution system in charge and discharge modes shows that the reliability effect of BESS is related to such factors as the load rate of the area which BESS is connected to, the reliability of the connected bus, and so on. When in the optimal allocation, BESS can improve the reliability of distribution system significantly.