The Energy Storage System (ESS) external characteristics of Vanadium Flow Battery (VFB) are studied, and the ESS is simulated based on the VFB electrochemical properties in this paper. Based on the systematic analysis of the actual operation data of ESS and the evaluation of the influence of the overload operation of ESS on the scheduling control, the function relationship is obtained by combining the key parameters such as State of Charge (SOC), electromotive force and equivalent resistance. On this basis, the Thevenin’s equivalent circuit is used as the main circuit of the simulation model, and the state parameter change is fed back to the main circuit in real time by means of controlled source and controlled resistance, and the running characteristics of the overload operation of ESS are embedded in the model. Finally, the proposed model is built and simulated by Matlab/Simulink. The comparison between the simulation results and the measured data show that the model can effectively describe the external characteristics of ESS and has high engineering application value. This work is supported by National Natural Science Foundation of China (No. U1766204), Science and Technology Project of State Grid Corporation of China (No. 2018ZX-10), and Science and Technology Project of the Headquarter of State Grid Corporation of China “Key Technology and Application of Super Capacity Battery ESS Grid-connected Operation”.