Abstract: In the variable polarity plasma arc welding (VPPAW) process of aluminum alloy, the controlled stability of variable polarity welding current is the premise to ensure the welding forming quality. However, the shape of plasma arc will change with the shear of welding current polarity, which makes the plasma arc as the output load of VPPAW power supply have significant nonlinear characteristics. Conventional PID control strategy cannot ensure the distortion of the output current waveform of VPPAW power supply, and the distortion of the output welding current waveform will further aggravate the nonlinear characteristics of the arc load and reduce the controlled stability of variable polarity welding current. In order to improve the controlled stability of variable polarity welding current of the VPPAW process, the controlled stability of welding current and the dynamic stability of plasma arc of aluminum alloy through hole vertical upward welding process under the conventional PID control strategy are studied, the fuzzy control theory is introduced, and then, a fuzzy PID controller with parallel computing ability is designed based on the parallel logic timing design method of field programmable gate array (FPGA). The verification test results show that the fuzzy PID controller can effectively improve the stability of the variable polarity welding current in the VPPAW process.