The optimal configuration of battery energy storage system is key to the designing of a microgrid. In this paper, a optimal configuration method of energy storage in grid-connected microgrid is proposed. Firstly, the two-layer decision model to allocate the capacity of storage is established. The decision variables in outer programming model are the capacity and power of the storage system. The objective is the least investment on the battery energy storage system. The decision variable in inner programming model is the charging and discharging power of battery. The objective is the lowest power fluctuation on the connection line. Then a case containing a grid-connected microgrid with wind power, photovoltaic, battery energy storage and load is studied, and the multi-scenario probabilistic method is used. The last result of energy storage configuration is calculated through the probability of each scene.