Abstract: The rules of arc shape and mechanical characteristic variated with the variation of the welding parameters and the mechanical characteristic of the variable polarity plasma arc were investigated.It was proved that the combined plasma arc tended to form the double arc,using the high-speed photograph and by detecting synchronously force and current of the arc.The experimental results had great indicative significances to the technology stability and process control in the variable polarity plasma arc welding on middle-thickness aluminum plate.