Abstract: A variable-polarity gas metal arc welding (VP GMAW) welding power supply based on STM32F405 is built to solve the problems of high-heat input and easy deformation for the metal sheet, which is applied to adjust the heat input through modulating the duration of EN/EP and the amplitude of output current. The mathematical model of a second inverting stage with coupling inductance is established based on the analysis of the commutation of the output current. Besides, ideal output waveforms and a control method are developed to solve some problems in the process of variable-polarity (VP) welding. On the basis of the research above, a simulation model for verifying the feasibility of the control method provided is carried out to work as a good reference model for the design of a VP power supply. Compared with the experimental waveforms, the results show it lives up to the factual expectation. Finally, an experimental prototype of VP GMAW power supply is developed to perform a bead welding experiment on an 1 mm thick metal sheet. The result shows that the weld surface is smooth and regular, and there is still an effective and stable welding progress in case of the commutation of output current polarity.