In the hybrid control of frequency-shifting and phase-shifting for full-bridge LLC converters, most designs use two variables for control. The choice of different control variable relationship curves will affect the power supply work efficiency. To optimize the power supply work efficiency of the hybrid control of full-bridge LLC converters, a design method of critical phase-shifting control curves is proposed. Verification is carried out through simulation and an experimental prototype of a 28 V 3.3 kW power supply. The experimental results verify that the control curves designed by this method can effectively reduce the switching losses of full-bridge LLC resonant converters in the hybrid control range and optimize the work efficiency of full-bridge LLC converters.