Abstract: High frequency isolated bidirectional DC/DC converter is an important hub equipment of urban flexible power supply system based on DC distribution. As a typical topology, CLLC converter has the advantages of high efficiency and strong power symmetry. However, due to its complex startup process, it needs to be analyzed and optimized when connecting to DC distribution system. In view of the fact that CLLC may be directly connected to the DC bus during startup, or live resistive or capacitive loads, the existing soft start methods are first studied. Then, a new soft start method with asymmetric duty cycle is proposed to solve the problems of the existing methods. Finally, different soft start methods are compared through Matlab/Simulink. Results show that the proposed method can effectively suppress the current spike at the starting moment, avoids the high frequency fluctuation of resonant current caused by charging and discharging of parasitic capacitors, and always ensures that half of the rectifier diodes can turn off ZCS. The proposed method can effectively guide engineering practice with its simple working mode and easy realization.