Abstract: Most of the current studies on the modeling methods of the energy storage system focus on electromagnetic simulation model,which are difficult to be used to simulate the impact of the accessed large-scale energy storage system on the system stability. In this paper,we propose an electromechanical transient modeling method for mass energy storage system,and build an electromechanical transient simulation model for energy storage system considering amplitude limit part and additional power control in the user-defined（UD）module of the Power System Analysis Software Package（PSASP）. Based on the electromechanical transient model, the adaptive additional power control method is proposed and the corresponding control module is built. The simulation results under different operating conditions prove that the proposed model can accurately simulate the transient support of the energy storage system to the power system,and the proposed adaptive additional power control method can further improve the transient support capability of the energy storage system.