Abstract: Energy storage technology, as an important element of the micro-grid, the rationality of its capacity configuration seriously impacts the operation performance and economy benefit of the micro-grid. This paper mainly focus on three purposes: Short-time power supply, Electric peak regulation & hot standby and Distributed generation power smoothing. And it proposed according to the main effect of BESS in micro-grid, with considering the requirement of the project on power supply reliability and power quality, and the operating condition of the battery, optimization calculation method on flexibly configuring BESS capacity, which provided theoretical basis of micro-grid project design.