The parameters of resonant elements in the CLLC resonant converter will affect the converter’s gain, volume and other characteristics, and the currently available parameter design methods have poor accuracy, complicated algorithms and other shortcomings. The feasible region under the constraint condition is determined by the visualization method, and the appropriate parameters of the converter are designed. The variable-frequency and phase-shifting control of the converter are analyzed in detail, and the system is modelled by the first harmonic approximation (FHA). As for the wide output gain, the method of variable-frequency combined with phase-shifting control is proposed. Both kinds of control can work in a soft-switching condition, which improve the converter’s efficiency. Finally, the simulation experiment is used to demonstrate this control strategy.