Abstract: Aiming at the problems of low load duration, single power output mode, and poor consistency of welding joints of traditional transistor resistance spot welding power source, a variable polarity transistor micro resistance spot welding power source which can realize fast charge and discharge of the capacitor is designed. The load characteristics of the power supply are analyzed. The main circuit topology of the power supply based on the full-bridge inverter + H-bridge dual-phase chopper circuit is designed. The digital power control system based on the high-performance STM32 dual-core controller is designed. A multi-stage variable pulse width capacitor group fast charging method is proposed. The power output characteristic test and the copper-nickel sheet single-sided double-point process test are carried out. The test results show that the designed power supply output load persistence rate can reach more than 5%, and the polarity switching time can be as low as 0.1 ms or less. The variable polarity output mode can effectively solve the problem of different solder joint sizes caused by the polarity effect in the unipolar mode. The performance of the welded joint in the voltage control mode is better than that in the current control mode and the power control mode.