Abstract: Abstract：This study focuses on the lap repair of cladding in spent fuel pool in nuclear power plants, employing an independently designed underwater laser welding torch. We conducted overlap fillet experiments on 3mm-thick 316L austenitic stainless steel plates at a water depth of 0.5m using an adjustable ring-mode laser system. As laser power increases, the throat size of the weld first decreases and then increases before decreasing again, the penetration depth of the base plate shows an increasing trend, the leg size of the weld gradually increases, the wetting angle gradually decreases, and the spreadability of the weld is improved. Under the experimental conditions, the weld metal exhibits a Ferrite-Austenite(FA) solidification mode, and the weld structure consists of γ-austenite and a significant amount of residual δ-ferrite. By adjusting the proportion of the ring laser, it is found that the central power has a significant effect on the growth of columnar crystals at the bottom of the melt pool, while the ring power affects the number of equiaxed crystals in the upper part of the melt pool. Increasing the proportion of ring power refines the equiaxed crystals at the center of the weld and makes the weld structure more uniform, thereby increasing the microhardness at the center of the weld. This study provides important process parameters and relationships between microstructure and properties for underwater laser welding repair of the bottom plate of spent fuel pool in nuclear power stations.