Abstract: This study addresses issues such as poor welding accessibility in the repair of thick-walled small-diameter stainless steel pipelines in nuclear power plants, focusing on the development of a TIG autogenous automatic welding process for Φ33.4×3.4mm 304L pipes in the 5G position. Through shielding gas penetration tests and considering automation requirements, the optimal gas mixture ratio of 30% Ar + 70% He was determined. By optimizing welding current, controlling back shielding gas pressure, and implementing multi-zone segmented regulation, issues such as insufficient penetration and denting in the pipeline were effectively resolved. Tests demonstrated that this process can accommodate assembly deviations of up to 0.8 mm misalignment and 0.5 mm gap. The weld seams passed visual testing (VT), penetrant testing (PT), radiographic testing (RT), as well as mechanical and metallurgical analyses including tensile, bending, and metallographic tests—all meeting the requirements of the RCC-M standard. This provides a reliable automatic welding process solution for pipeline repair in nuclear power plants.