Abstract: The variable polarity keyhole plasma arc vertical welding process characters of aluminum alloy were analyzed and the method was found that the variable cross-section aluminum alloy was welded with the variable polarity keyhole plasma arc vertical welding by precisely controlling welding parameters.Welding current,the flux of plasma gas and welding speed were defined as the adjusted parameters.The dynamic balance of thermal and force in keyhole weld pool is the key to regulate the welding parameters and realize the automatic welding of variable cross-section sample.The control device of the VPPA welding system was constructed based on 80C196KC.So it can regulate welding parameters in real-time and keep the balance between thermal and force dynamically and realize the variable polarity keyhole plasma arc vertical welding process of variable cross-section aluminum alloy.