Abstract: S32101 duplex stainless steel is the cladding material for the spent fuel pool of third-generation nuclear power plants. After long-term service, cracks are generated due to corrosion, which affects safe operation and requires welding repair. Cracks were removed by opening grooves and repair was carried out by filling and welding the grooves. Taking the crack repairing of S32101 duplex stainless steel as the goal, filling welding repair experiments of a 6 mm deep U-shaped groove were carried out in an underwater environment with different laser powers. The surface of the welds obtained by filling welding with different powers is well formed, but there are pore like defects at the bottom of the 5000 W and 5300 W filling welds, and there are no defects such as pores inside the 5600 W and 6000 W filling welds. XRD, EDS, and optical microscopy were used to analyze the microstructure, chemical composition, and phase composition for the defect free weld seam. Tensile test and hardness test were conducted, the electrochemical corrosion behavior of welds was studied by using activated potential polarization and AC impedance spectroscopy tests. The results indicate that the microstructure of the weld is composed of Weinstein austenite, intragranular austenite, grain boundary austenite, and ferrite(δ). The rapid cooling effect of underwater environment has a certain inhibition effect on the growth of austenite phase. The grain size of γ(111) is less than that of δ(110). The high content of Ni and Mo in the welding wire composition, as well as the addition of N element in underwater environmental protection gas, play an important role in improving the corrosion resistance and mechanical properties of the weld seam. The weld seam after underwater environmental filling and repair has good intergranular corrosion resistance.