The main theme of this paper is to propose a new control method of optimizing the switching frequency to reduce the loss of phase-shifted full-bridge (PSFB) converter. The detailed loss analysis of PSFB converter is performed and the loss analysis model is given. Based on the model, the optimum switching frequency that will result in minimum total power loss under variable load conditions is studied. The controller adjusts the converter switching frequency according to the optimum switching frequency curve. Experimental results show that the efficiency improvement can be up to 15% under light-load condition. These results confirm the effectiveness of the proposed control method of optimizing the switching frequency.