In order to implement the GMAW automatic welding of steel bar piercing plug weld under inclined position in the shielding building of a third generation nuclear power plant,the effects of welding path,welding posture and welding gas selection on weld formation are studied through experi-ments in this paper. The results show that 360 degree alter-nation and refining layers can effectively control the flow of molten pool to ensure the weld reinforcement within 2 mm;When the working angle of the welding gun is adjusted to 15°and the welding direction angle is 6° ～10°,the arc force can effectively offset the tangential component of gravity and effectively avoid the lack of fusion for the side-wall and the interpass;compared with φ （Ar）98%+φ （O,2,）2% mixed gas,φ（Ar）80% +φ （CO,2,）20% ratio can significantly increase the surface tension of molten pool metal to ensure the quality of weld formation.