Abstract: Taking the underwater welding of positioning pin in nuclear power plant as the research object, an underwater TIG welding gun with double-layer gas protection is developed. The motion trajectory is realized by DC motor driving the tungsten pole to rotate around a fixed diameter. The nuclear grade material Z2CN19-10 nitrogen-controlled stainless steel was welded. The weld formation of local dry underwater TIG welding was studied. The process parameters were optimized. The microstructure and mechanical properties of the joint were analyzed by combining the thermal cycle curve and arc shape. The results show that when the inner and outer layers are filled with argon, the weld is well formed and the arc shape is stable. Increasing the welding current or reducing the welding speed increases the weld penetration and the weld width. By comparing the underwater and onshore welded joints, it is found that the rapid cooling of water will promote the ferrite near the fusion line to change from dendritic to lath, reduce the austenite content and refine the grain. The microhardness and mechanical properties of underwater joints are slightly higher than those of onshore joints.