This study investigates the influence of different layers and welding passes on the macroscopic morphology, microstructure, mechanical properties, and corrosion resistance of multi-layer and multi-pass fillet welds in the context of repairing cladding defects at the corners of spent fuel pools in nuclear power plants. The results show that two-layer two-pass and three-layer five-pass welding can effectively increase the thickness of the fillet weld to 3.6 mm and 5.7 mm, respectively. Meanwhile, two-layer three-pass and three-layer six-pass welding can increase the size of the weld foot to 6 mm and 9 mm, respectively, based on the increased thickness. In terms of microstructure, ferrite is distributed in the weld with varying morphologies: skeleton-shaped or worm-like ferrite is predominantly found in the weld center, while tree-like ferrite is mainly present on the weld surface. Additionally, ribbon-like ferrite is observed at the junction of the bottom plate and the vertical plate. As the welding sequence increases, coarse grains are refined, and fine tissues undergo secondary growth, resulting in differences in the size and growth direction of the weld tissues. Regarding mechanical properties, the measured microhardness of the fillet welds exceeds that of the base material. In terms of corrosion resistance, two-layer two-pass and three-layer five-pass welds exhibit severe corrosion after pitting tests, with uniform corrosion and oxidation zones appearing on the surface. In contrast, two-layer three-pass and three-layer six-pass welds demonstrate good corrosion resistance, with no obvious pitting pits. These findings provide guidance for multi-layer and multi-pass fillet welding.