Abstract: VPPA pressure was measured at different welding parameters by U-tube barometer method.The effects of different welding conditions on arc force, including straight polarity current, reverse polarity current, time ratio, plasma gas flow rate were analyzed by the orthogonal experiment.The results indicates that VPPA pressure increased with these four parameters, and the influences of the welding parameters are in order of plasma gas flow rate, straight polarity current, time ratio, and reverse polarity current.Then the mechanisms were discussed.The plasma gas can cool and compress the arc column and compress the arc radius, which increases the VPPA pressure;while the welding current increases the arc radius by electromagnetic compression, which enhances the VPPA pressure.Moreover, due to the great difference of VPPA pressure between straight polarity phase and reverse polarity phase, time ratio is also one of key factors to VPPA pressure.