Abstract: Underwater local dry welding technology has significant potential for applications in marine engineering, ship repair, and subsea pipeline maintenance. However, the fumes generated during the welding process substantially reduce visibility inside the drainage chamber and negatively influence weld pool observation and visual weld tracking. Reducing the fume concentration within the chamber is therefore essential for the practical implementation of this technology. In this study, numerical simulations were conducted to model the internal flow field during the drainage and fume exhausting processes. The effects of various combinations of drainage and fume exhausting vents on the efficiency of drainage and fume removal were analyzed. A laser transmittance method was used to quantify fume concentration and detect the fume exhausting during underwater local dry welding. Simulations of the fume exhausting behavior were compared and verified with the test results. The results show that, through optimization and comparison, while ensuring effective drainage of the drainage chamber, the reasonable addition of dedicated fume exhausting vents effectively reduces fume concentration during welding by about 62% compared to that without a dedicated fume exhausting vent and increases visibility inside the chamber to over 80%. As a prerequisite for observing the underwater local dry welding process and conducting visual tracking of the weld, this provides technical support for the engineering application of underwater local dry welding.