The CLLLC resonant converter inherits the soft switching characteristics, impedance characteristics and gain characteristics of the LLC resonant converter in forward and reverse directions. There are problems and difficulties in the start-up of CLLLC resonant converter, and it has more complex gain characteristics and impedance characteristics than the LLC resonant converters. By discussing the factors affecting the current and voltage stress at the start-up moment and during the start-up process, the different soft-start strategies are proposed in the paper. The feasibility of the proposed soft-start strategy is proved by comparing the start-up current and voltage stress with the current and voltage stress at the steady-state resonant point in the simulation waveform.