Aiming at the problems of wide frequency adjustment range and low operating efficiency of the traditional variable frequency control CLLLC resonant converter at low voltage gain, a variable mode resonant converter based on the model predictive control is designed. The control strategy can make the converter switch between the full-bridge mode and half-bridge mode flexibly in a wide range of input without additional auxiliary circuits, which can broaden the low voltage gain and keep a narrow frequency adjustment range. This strategy is beneficial to the design of magnetic components and effectively improve the operating efficiency of the converter under high input voltage. Compared with the PI control, the proposed model predictive control obviously improves the output voltage spike problem during mode switching, and increases the dynamic performance of the converter on the premise of similar steady-state performance. Simulation and experiment prove the feasibility and superiority of the designed CLLLC resonant converter and its control strategy.