The invention discloses a variable polarity plasma arc welding method for a multi-axis linkage space curve welding seam. The variable polarity plasma arc welding method comprises the steps that firstly a workpiece is cleaned before welding, and after assembling the welding seam position of the workpiece, a welding tool is used for installing the welding seam position to a biaxial positioned; thena welding robot system is installed on a three-axis moving linear guide rail device, the three-axis moving linear guide rail device can independently adjust in the height direction, the horizontal direction and the vertical direction, the biaxial positioner can realize the double-degree-of-freedom overturning displacement of the workpiece, and the three-axis moving linear guide rail device and thebiaxial positioner realize the real-time control over the relative position of a welding gun installed at the tail end of the welding robot system and the workpiece welding seam on the welding tool;and finally the welding seam welding track pilot run is performed, deviation is corrected and welding parameters of the welding robot system are adjusted, and workpiece welding is completed.