Abstract: Objective Underwater local dry welding is an effective means to achieve high-quality and efficient underwater welding. However, it is difficult to ensure complete drainage of moisture in the local dry area. To address the problem that residual moisture can lead to poor weld formation, the mechanism of residual moisture and the welding temperature field under unclean drainage are studied. Methods Using 6 mm thick 304L stainless steel as the test material, TIG lap tests are conducted under different residual moisture conditions (with/without water film) in the local drying area. After welding, macromorphology welds are observed and metallographic specimens are made. The cross-sectional formation and microstructure of the weld is observed under a microscope. Finite element models, temperature field models and convective heat transfer models are established under different environmental humidity levels. The temperature field during the welding process is simulated. The accuracy of the temperature field is verified. Moreover, the calculation results of the temperature field is analyzed. Results The results indicate that, under appropriate welding parameters, welds with good surface quality and fewer internal defects can be obtained in both locally dry areas with/without water film. The difference is that the cross-sectional size of the weld with water film is slightly smaller than that without water film. The peak temperature of the simulated temperature field is located in a reasonable range of arc welding peak temperature (1700～2300 ℃), and the maximum simulation error of the weld section is 7.5%, indicating that the simulated temperature field has high reliability. The temperature change curve of the observation point shows that the peak temperature at the center of the weld is 121.68 ℃ higher and the cooling rate is 13.63 ℃/s slower when there is no water film than when there is water film. The residual moisture in the dry area boils and evaporates under the action of welding heat input, which increases energy loss and reduces energy utilization efficiency during the welding process. At the same time, the water vapor is generated by evaporation increases the air humidity, thereby enhancing the convective heat transfer during the welding process, further increasing the welding energy loss, and ultimately leading to a decrease in the fusion width and depth of the weld cross-section. Conclusion When the drainage in the local dry area is not clean, the less residual moisture still has a significant impact on the welding process and welding results. This article only focuses on the mechanism of its impact on weld formation, it is expected to provide data reference and theoretical support for in-depth research on unclean drainage.