Abstract: High-quality joining of low-alloy steel and austenitic stainless steel is a critical step in the fabrication of nuclear power nozzle safe ends. Significant differences in the physical and chemical properties of these materials, together with the thermal cycles associated with narrow-gap multi-pass welding, can exacerbate interfacial microstructural mismatch and stress concentration, thereby affecting the overall performance of the joint. To address this issue, an oscillating laser–tungsten inert gas (TIG) welding twin-wire hybrid welding technique was developed in this study. High-quality narrow-gap joints of 40 mm thick SA508Ⅲ/316LN dissimilar steels were fabricated using Inconel 52M nickel-based alloy as the filler metal and Inconel 690 as the transition layer. In terms of macroscopic formation, the narrow-gap joint is free of macroscopic welding defects, and continuous metallurgical bonding is formed at the dissimilar metal interfaces. Microstructural analysis reveals that the Inconel 690/SA508Ⅲ interface is an incoherent heterophase interface with an insufficient elemental transition zone. Pronounced mismatches in crystal structure and hardness exist at this interface, and local stress concentration occurs on the SA508Ⅲ side, making it the weakest region of the joint. The joint achieves tensile strengths of 609 MPa and 475 MPa at room temperature and 350 ℃, with corresponding elongations after fracture of 24% and 28%, respectively. The low-temperature impact energy reaches 202.4 J, and the fracture surfaces exhibit typical ductile fracture features. This study provides a novel technical route for efficient and high-quality narrow-gap joining of dissimilar steels.