The research of gas shielding stability technology, playing a key role in underwater local dry welding, was introduced in this paper. The study includes shielding cup design, gas flow simulation, draining and welding experiments. The commercial computational fluid dynamics(CFD) software FLUENT was applied to get and compare the speed and pressure contour images of gas in the three kinds of cup with different intake mode. The computed results show that the cup with stilling chamber on the top has the best shielding performance. The underwater welding experiment of 15 meters proves that the shielding cup with stilling chamber can offer a good dry space, the welding arc can burn stably in it and the weld quality is perfect. The research will facilitate the application of underwater local dry welding technology in the maintenance of nuclear power station widely.