This paper constructed a model of VRB (all-vanadium redox flow battery),which is suitable for grid simulation.The external characteristics of VRB energy storage system were researched.Then an equivalent model of VRB energy storage system was proposed from the principle of external characteristic equivalent fitting and ignoring the reactive power of the VRB system.This equivalent model has a simple structure, less parameters,and can be identified easily.It can fit the external characteristics of VRB energy storage systems effectively in the case of large disturbance and continuous small disturbance of power system． The advantage is that it can simulate the output limit of the battery energy storage.A GSLM (Generalized Synthesis Load Model) was established in which the proposed VRB equivalent model is connected in parallel with the traditional load model considering distribution network parameters and reactive power compensation, the generalized synthesis load model multiplied with equivalent model is used to fitting the characteristics of power synthesis load containing the VRB energy storage systems.The examples show that, compared with the traditional integrated load model,the GSLM can precisely describe the integrated load characteristics of the distribution network with energy storage system,the greater the proportion of VRB in the synthesis load,the more obvious this advantage,and the parameter identification results of the model have good stability and comprehensive description ability.