Proposed in this paper is a STM32-based LLC resonant charging power supply system for electric vehicles,which employs digital PFM ( Pulse Frequency Modulation) for the system control and adopts an LLC resonant topology as well as a four stages + positive and negative pulse charging scheme that meets the charging requirements of lead-acid battery.Test results demonstrate that ( 1) the proposed power supply system helps to implement the whole-course soft switching state of the main circuit of LLC resonant converters; ( 2) the main circuit of LLC resonant converters always operates in the primary-side MOSFET ZVS and the secondary-side rectifier diode ZCS softswitching state,which improves the power conversion efficiency; and ( 3) the combination of battery charging curves and four stages + positive and negative pulse charging scheme can reduce the overcharging as well as gassing polarization,protect the battery and increase the charging speed.