Abstract: Based on the relevant studies,in order to bring the battery energy storage system economical benefits in the user side caused by reducing capacity of user’s distribution station and decreasing the power expenses for user,a value model for reducing loss of the transformer and power outage cost is built.In the mean time,considering the investment cost and operation and maintenance cost,the capacity optimization plan model for user’s battery energy storage system is developed and particle swarm optimization algorithm is used to solve it.