Abstract: According to the brazing techniques of large-scale generators and high power transformers, a 20 kHz/40 kW superaudio frequency inverted welding power supply was dasigned. It used PSM (Pulse-Symmetrical Modulated) power control inverter. The serial-resonant full bridge inverter executes both power converting and power adjusting. The main switches of the inverter are controlled according to the PSM and the inverter always works at resonant with the same pace of the load and work in shutting off at ZCS and ZVS while switching on and off. The inverter is with unity power factor and raises efficiency.