In order to apply the gas metal arc welding（GMAW） with high current to industrial production,an alternating magnetron power supply is developed,which acts on the excitation coil coaxial with the welding torch and generates a vertical alternating magnetic field,so that the rotating spray transfer with high current could be controlled to increase welding efficiency. The power supply uses 16-bit 80C196 KC MCU as the main control chip and the main circuit adopts double inverter structure. The primary inverter has a half-bridge inverter structure to realize constant current output and the secondary inverter controls output square wave AC frequency and duty ratio. The magnetron power supply with a digital human-machine interface has protection functions such as overheating,overcurrent,overvoltage and undervoltage to ensure the reliability of the power supply. The results indicates that the developed magnetron power supply could produce alternating current with adjustable excitation current,frequency and duty cycle,which acts on the excitation coil to control the unstable rotating spray transfer.