Abstract: The variable polarity plasma arc welding of 6mm thick 2A14 aluminum alloy sheets was carried out.The reason for defects occurred during horizontal welding was clarified through analyzing the forces acting on the welding pool.The difficulties in VPPA horizontal welding were formation of undercuts on both sides and the gathering of blowholes around the boundary in upper part of the weld.It was found that less welding heat input was beneficial for avoiding the undercut on top side,but larger welding heat input was beneficial for reducing undercut on bottom side.The defects during horizontal welding might be attributed to the fluid flow caused by gravity or the balance of the forces acting on the welding pool.