Abstract: A new type of variable polarity welding power source applied to Al alloy TIG welding has been introduced in this paper. The power source mainly consists of a constant current-controlled transistor and a IGBT H bridge inverter.The amplitudes and durations in both positive and reverse cycles can be adjusted as operators' wish.The pwoer capacity of the transistor set needed in this new power supply is only a third as much as that of the conventional process since the special design and structure are used. The open circuit voltage is only 40 volts, but no pulsed arc stabilizer is needed and the welding process still stable, especially in low welding current.