Due to flexible location, short construction cycle and rapid cost reduction, chemical energy storage has been applied to power grid peak load regulation in recent years. However, re-search on factors sensitive to optimal capacity of battery energy storage system suitable for peak load regulation is still insufficient.A model about optimal capacity of battery energy storage for peak load regulation is constructed. The model takes lowest social energy consumption cost as a goal, and planned or forecasted data of allkinds of power source and load in target planning year as boundary condition. The model makes full time production simulation foreach alternative capacity scheme of battery energy storage through-out the year, and solves it by mixed integer linear programming.Based on the work above, the sensitive factors that affect the optimal capacity of battery energy storage are summarized, and empirical research and analysis on the mode and degree of its influence are carried out. The results show that the energy storage factor, including unit investment cost and operating years, as well as demand factor such as peak load supply and low load adjustment, caninfluence the optimal capacity of battery energy storage and the total system cost.