Abstract: To address the issues of insufficient impact toughness of the weld metal and low welding efficiency of the traditional arc welding in Pelton turbine runners, a narrow-gap laser-TIG hybrid welding process was proposed in this paper. The influence of process parameters on the weld formation quality in narrow-gap grooves was investigated through a single-factor controlled variable method, and the microstructure and mechanical properties were analyzed. The results indicate that the weld exhibits good formation without defects when the laser power is 3000 W; the welding current is 320 A; the welding speed is 0.2 m/min; the wire feeding speed is 5 m/min; the oscillation angle is 10°, and the deposition efficiency is 2.7 kg/h. By using these optimized welding process parameters, high-quality welding is achieved on a 50 mm-thick super martensitic stainless steel mock-up. Different regions of the weld exhibit distinct microstructural characteristics due to the variations in thermal cycles, and the middle region forms a uniform and fine equiaxed grain structure after experiencing multiple thermal cycles and tempering effects. The microhardness of the weld fluctuates in the range of 303 ~ 320 HV10; the yield strength reaches 937 MPa ± 58 MPa, while retaining good plasticity. The overall impact toughness of the weld is excellent, and the impact absorbed energy at 0 ℃ is 303 J ± 21 J. In summary, the narrow-gap laser-TIG hybrid welding technology can significantly improve the welding quality and efficiency of the runner, providing a new technical path and practical guidance for the welding operations of large-scale runners.