Abstract: The high-frequency isolated bidirectional DC-DC converter is a key component of cascaded battery energy storage system,which should meet the requirements of wide voltage gain range and high efficiency.In the cascade battery energy storage system,aiming at the problem of efficiency reduction of dual active bridge（DAB）under wide gain range due to backflow power and high current at switching time,on the basis of three phase shift（TPS）modulation strategy,an optimized modulation strategy based on interior point method is proposed,which takes the total loss as the optimization objective. The proposed strategy constructs a unified loss model of DAB under different gains. On the basis of ensuring the soft switching（ZVS）of the switching devices,the interior point method is used to obtain the optimal value of phase shift angle according to different battery open circuit voltage（OCV）. The influence of inductance value and switching frequency on the average efficiency of the whole voltage gain range is analyzed,which provides a reference for DAB parameter design. A 5 k W low-power experimental platform was constructed to verify the effectiveness of the optimized modulation strategy.