With the increase of switching frequency, the switching loss in LLC resonant converter will increase. Gallium nitride (GaN) device with high frequency characteristics is applied to LLC resonant converter with planar transformer and synchronous rectifier technology to achieve the double improvement of switching frequency and efficiency. Firstly, the working principle of LLC resonant converter was analyzed. Then, the resonant element parameters were designed to reduce the loss. On this basis, two sets of control experiments were established, and the losses in the power tube, transformer and rectifier tube in the two experiments were calculated. Finally, two experimental prototype platforms were built. Relevant experiments show that the efficiency is increased by 11.25%, which verifies the rationality of the design scheme.