Abstract: To achieve high-quality crack repair in spent fuel pools and address the difficulty of water drainage from underwater grooves, optimization experiments for the underwater laser welding of S32101 duplex stainless steel U-shaped grooves were conducted using three drainage processes: high-flow drainage, groove gas purging, and synchronous groove gas purging. The results indicate that the synchronous groove gas purging drainage process is the optimal approach. The resulting welded joint exhibits a distinct fusion line and is free from lack-of-fusion and porosity defects. The microstructure primarily consists of intragranular austenite and grain boundary austenite. Under the influence of multiple thermal cycles, secondary austenite precipitates in the filling and root zones, and equiaxed ferrite is observed in the root zone. The transverse hardness profile of the joint displays a double-hump shape, and the average hardness of the weld is 33 HV higher than that of the base metal. The longitudinal hardness profile shows a step-like distribution, with a peak hardness of 331.1 HV appearing in the laser remelting zone of the 3rd filling layer. In the tensile tests conducted at 20 ℃and 130 °C, all specimens fracture in the base metal, demonstrating that the tensile properties of the joint surpass those of the base metal. In the impact tests conducted from −60 ℃to 25 °C, the absorbed impact energy of all specimens exceeds 6 J. Bending tests performed on the specimens before and after the intergranular corrosion test reveal no cracks, indicating that the bending and intergranular corrosion resistance properties are qualified. The comprehensive properties of the welded joint under this process meet the construction standards for spent fuel pools of nuclear power plants.