Abstract: Taking the repair of defects at the corners of the spent fuel pool cladding plate of the nuclear power plant as the background, the local dry underwater laser fillet welding test platform was used to carry out the local dry underwater laser fillet welding test of multi-layer multi-pass fillet welding with filler wire. The results show that: compared with the fillet weld in the air environment, the underwater fillet weld is well formed, with a more bright surface, and there are no defects such as cracks or undercuts; in terms of the microstructure, the equiaxial crystals of the underwater fillet weld are reduced and the cellular crystals at the intersections with the base material increase. The ferrite exists in the form of dendrites and laths; the microhardness of underwater fillet weld is higher than that of air fillet weld. Combined with the EDS analysis, it is found that the rapid cooling of water leads to elemental segregation, making the microstructure distribution not uniform; the ferrite content attached to the austenite matrix inside the underwater fillet weld is small, making the dendritic crystal inside the weld and the columnar crystals have similar directionality, which promotes the pitting corrosion process to a certain extent; and the rapid cooling can form the compressive layer, offsetting the original tensile layer, thus reducing the residual stress of the weld, and increasing the residual stress of the weld, but also improve the corrosion resistance of the underwater weld, under the joint effect of the two, the overall corrosion resistance of the underwater fillet weld is similar to the dry fillet weld. The results show that the fillet weld prepared by 304 stainless steel local dry underwater laser filler fillet welding meets the service requirements.