Abstract: Taking the keyhole molten pool in plasma arc welding as the study object,the weld formation in variable polarity keyhole plasma arc transverse welding of aluminum alloy was investigated by using both experiments and numerical simulation according to the characteristic of heat source.The study indicates the deviated phenomena between the highest temperature point and the widest fusion width section in keyhole molten pool root surface under the heat source due to the welding speed fluctuation and effect of thickness of plate.By making division of the keyhole molten pool root surface,the concept named temperature width deviation was defined as the deviation between the highest temperature point and the widest fusion width point in molten pool root surface,to describe the state of keyhole molten pool root surface and the welding formation.Temperature width deviation was changed by adjusting the welding torch position,so as to reduce the effect of gravity in welding process,with other parameters being unaltered,to get a good weld formation of variable polarity keyhole plasma arc welding at transverse position.