In order to optimize the capacity allocation of the energy storage system, improve the economy and energy saving effect, and realize the peak shaving and valley filling of the system, based on the MATLAB platform, an optimal allocation model of energy storage capacity considering flexible load was built. Meanwhile, a comprehensive evaluation method was proposed considering multiple optimization objectives such as the matching degree of system source and load, the smoothness of load curve, the comprehensive energy utilization rate and the comfort of electricity consumption on the basis of the lowest investment and operation cost and the least carbon emissions. The results of Nanjing Jiangbei Energy Station show that the flexible load adjustment can not only improve the overall operation economy and environmental effectiveness of the system, but also improve the comprehensive energy utilization rate and achieve the purpose of peak shaving and valley filling under the premise of ensuring comfort, realize the friendly interaction between source and load, and alleviate the pressure on the power grid.