Abstract: The output voltage stability of high voltage charging power supply is the key technical index of pulse modulator. It is necessary to study and improve the output stability of high voltage charging power supply. Based on the working principle of LLC resonant converter composed of magnetizing inductance, resonant inductor and resonant capacitor, the voltage gain curve of LLC resonant circuit was obtained according to LLC equivalent circuit, the characteristics of LLC resonant converter working mode were analyzed, and the selection criteria and design method of planar insulated core transformer were proposed. The inductance parameters of transformer were obtained by measuring the actual transformer. The resonant frequency and the working frequency values of LLC resonant converter were selected, and the ripple amplitude of output voltage of high voltage charging power supply was calculated in detail according to the circuit principle of LLC resonant converter. The stability result is better than the design index of high voltage charging power supply. The experimental results show that the LLC resonant converter with laminated magnetic core meets the design stability requirements.