Abstract: Variable polarity plasma arc welding, an efficient and cost-efficient welding technology, has been widely applied to aircraft components manufacture, especially aluminum alloy components. Variable polarity plasma arc keyhole weld of 2A12 aluminum alloy was conducted by welding torch with assistant nozzles under high welding speed and high heat input, and microstructure and mechanical properties of weld zone was analyzed as well. Due to the compression of assistant nozzles to plasma arc, crown weld is narrowed, however, root weld is broadened; reinforcements of both welds are increased. In the weld, fine equiaxed nondendritic grains are dominate, strengthening phases are with Al₂Cu phases largely and few S phases. Mechanical properties of weld zone are determined by the size and quantity of Al₂Cu. It could be concluded that the difference of hardness between parent metal and weld is little. Furthermore, microhardness distribution curve alongside longitudinal direction of weld is like U-shape, plain in the middle of weld and fluctuating in both sides of the crown and root weld.