Abstract: The output power of variable polarity TIG welding power supply was studied. The results show the output power changes with the period and duty radio of variable polarity welding, and the output power in electrode positive is far large than that in electrode negative. The periodically changing output power will cause the variable polarity TIG welding power supply to absorb power from power grid periodically. The input current of the power supply was distorted seriously by using normal PFC(power factor correction) circuit and control strategy. A new PFC control strategy based on load observer is presented to reduce the harmonics of variable polarity inverter arc welding power supply. Experiments result shows that this control strategy is available and near unity power factor is obtained.