The technological characteristics of variable polarity plasma arc welding（VPPAW） of aluminum alloy are analyzed. The penetrating variable polarity plasma arc butt girth welding experiment of anti-rust aluminum alloy 5 A06 with thickness of 7 mm is carried out by using ER5356 welding wire.The influencing factors of weld forming quality are analyzed,and the application of variable polarity plasma arc welding to shell-flange girth welding in an underwater product is realized.The welding process is divided into six stages: arc-striking,perforating, welding, lapping, hole-closing and arc-extinguish.The quality and mechanical properties of the joint are tested by X-ray diffractometer and universal stretcher. The results show that the girth weld of 5 A06 aluminum alloy shell and flange is well formed. The weld quality meets the requirements of grade I weld of GJB 294 A-2005 "Fusion Welding of Aluminum and Aluminum Alloys". The tensile strength of the joint is 301 MPa,which is 90.2% of the base material. The fracture position is near the bond line. The mechanical properties of the joint are good and meet the requirements of the product.