Hybrid energy storage system（ESS）has multiple advantages and leads to an important ESS research direction in the future.Take valve regulated lead acid（VRLA）battery and vanadium redox battery（VRB）hybrid ESS as an example，which has massive application feasibility，a novel capacity optimization method for hybrid ESS is proposed.Firstly decompose fluctuation smoothing target according to different ESS medium;secondly construct charge and discharge strategy for both independent and complementary fluctuation smoothing;finally develop capacity optimization modeling taking economic operation as objective function，and realize calculation based on particle swarm optimization（PSO）algorithm.The actual wind farm power data shows that this method can optimize the hybrid ESS on equivalent running cost，smoothing effects，charge or discharge times，and has high validity.