Abstract: Objective 304L stainless steel is widely used in the repair of nuclear power plant spent fuel pool and it usually adopts underwater local dry welding. This paper aims to study the influence of high pressure and high humidity environment on weld formation, microstructures and mechanical properties of welded joints. Methods Using 6 mm thick 304L stainless steel as the experimental material, this paper conducts underwater local dry TIG lap welding tests under different pressure and humidity conditions in a self-developed pressure vessel. Results The results show that well formed weld without defects such as pores, inclusions and cracks under appropriate welding parameters. As pressure and humidity increase, the degree of weld oxidation increases and color becomes darker. The degree of oxidation intensification is higher when humidity increases than pressure. The cross-section of the weld is clean and flat, and the internal contour appears “elliptical arc”. Under normal pressure, the lower part of external contour protrudes, and the upper part penetrates upper plate. As pressure increases, the lower part of external contour becomes flat, and the upper part is not fully welded to upper plate. Pressure has a higher impact on weld depth with a maximum increase of 44.2%, while humidity has a higher impact on weld width with a maximum decrease of 27.6%. The austenite in weld zone is transformed from block in the base metal to sheet with smaller size, and much acicular ferrite is generated at the same time. Conclusion The acceleration of molecular motion under pressure and the intensification of convective heat transfer under humidity both lead to an increaseing cooling rate during the welding process, resulting in an increase of ferrite content in weld. The increase in ferrite content caused by pressure is greater than that caused by humidity. The microhardness of weld zone is higher than that of base metal, and the welded joint is pulled apart at the weld. With pressure and humidity increasing, the ferrite content in weld increases, the austenite content decreases, and the grain size decreases, resulting in an increase in the hardness and tensile strength of weld, and a decrease in the corrosion resistance of weld.