Abstract: The quality of automatic keyhole-mode vertical welding of thick aluminum alloy plate with variable polarity plasma arc welding process is severely affected by the large welding current and burning of tungsten electrode during long-period working.A new method was proposed with AC-DC mixing output current to reduce the burning rate of tungsten electrode and improve the welding quality.In ensuring cleanup of oxide film,this method could decrease the decline of arc force caused by twice crossing zero welding current within a very short time and decrease the damage to switching devices from the excessive induced electromagnetic force.Meanwhile,the mixing output current could realize welding pool oscillation and grain refinement by the low frequency modulation.The mechanical properties of plasma arc,welding process stability and mechanism of weld formation were also investigated.After examination of the weld microstructure and weld formation,it was found that the AC-DC mixing output could refine the weld grains,reduce the burning rate of tungsten electrode and improve the weld quality.