Abstract: Welding arc load characteristics of variable polarity TIG welding was studied through experiment.The experimental results indicate that equivalent resistance of welding arc in EN is less than it in EP, both of them decrease as increasing of welding current, the equivalent resistance will drop rapidly at the time when EP commutate to EN.The mechanism of equivalent resistance analyzed is concerned with the working mechanism of negative, arc column and anodic at different load current.A control strategy incorporate variable parameter PI into advance control is presented to solve the problem of vary equivalent resistance at the time EP commutate to EN.Experimental results prove this control strategy is available.