In order to further improve the power density and efficiency of the battery side bidirectional DC-DC converter in the energy storage system, a novel soft switching bidirectional DC-DC converter based on LLC resonance is proposed. The converter effectively reduces the transformer turns ratio and improves conversion efficiency. The converter can implement bidirectional LLC characteristics in an asymmetric half-bridge topology, and all switches in the converter can be soft-switched. At the same time, the converter has a simple structure and can adopt synchronous rectification technology, and has the advantages of high efficiency and low cost. The realization process of the converter soft switch is introduced, and the resonance characteristics and related parameters of the soft switch are analyzed. Finally, a simulation model was built with a high voltage side of 350 ~ 400 V and a low voltage side of 45 ~ 50 V, which verified the effectiveness and practicability of the converter.