Abstract: Objective Taking the leakage patch welding of nuclear power plant as the research object, underwater laser lap welding process was explored to realize in-service maintenance of key parts of nuclear power plant. Methods The form of oscillating laser heat source was used to study weld formation of local dry underwater laser welding, welding parameters were optimized, and microstructure of welded joints was analyzed by combining characteristics of thermal cycle curve. Results The results showed that with the increase of laser power, penetration and width of weld increased, and root discontinuity decreased first and then increased. At the same time, deep penetration welding led to pores. The transverse swing laser could increase the scope of laser beam, increase the bonding area of weld, inhibit welding pores and improve quality of weld formation. Compared with on-land welding, peak temperature of thermal cycle was significantly reduced and cooling rate was significantly increased in underwater laser welding, which led to higher ferrite content of underwater laser welded joints than that of on-land welded joints. Ferrite spindle was elongated, which basically ran through the whole weld. Conclusion Underwater welding of 304 stainless steel lap joints with oscillating laser heat source had obvious advantages.