CAP1400 nuclear power plant CV module steel plate（SA-738 Gr.B） has a small gap and automatic welding with a small filling volume and high welding efficiency. First a single variable experiment is conducted to study the influence of parameters such as swing speed,sidewall dwell time,welding current,welding speed,and the distance from the tip of the wire to the sidewall on the weld penetration depth,residual height,and sidewall penetration depth in a narrow gap groove,with a special narrow gap welding gun. The results show that in a narrow gap bevel,the swing speed has the greatest effect on the penetration of the weld;the welding current and the welding speed have the greatest effect on the remaining height;the distance from the tip of the wire to the side wall has the largest effect on the penetration,and determined the optimal welding parameter range. Second,the optimal parameters are used to weld the test plate,and the test plate is tested for mechanical proper-ties. The results show that the tensile properties and impact properties are higher than the requirements of the base metal.The study provides theoretical basis and reference for the further engineering application research of CV narrow gap automatic welding process.