DC solid state transformer is one of the core components in DC power grid, and its efficiency is an important index. The efficiency of a DC solid state transformer cascaded with resonant DC-DC converters as the basic unit is mainly determined by the resonant DC-DC converters. This paper analyzes the working stages of series resonant converter and discusses the main factors that affect the turn-off loss. The zero voltage switching-on (ZVS-on) condition of the circuit is derived. The scheme of improving the efficiency of the converter by optimizing the phase lag angle and resonant frequency is discussed. An experimental prototype of series resonant converter is built, and the experimental results preliminary verify the effectiveness of the efficiency optimization scheme proposed in this paper.