Abstract: In underwater local dry welding, the stability of welding procedure is determined by the drainage effect of local drainage device and the turbulence state of fluid in the device, which are two key factors that affect the welding quality. A local drainage device with a cylindrical drain cover structure was developed. Based on fluid mechanics, the simulation was carried out with software FLUENT to explore the drainage process in a circular drainage cover. The influences of different air inlet modes and drainage modes on the phase distribution in the cover and the turbulence state of fluid on the workpiece surface were analyzed. The accuracy of simulation was verified by comparing the simulated drainage process with the experimental one. The results show that the simulated drainage process was consistent with the actual process. Through comparative analysis, the reasonable air inlet and drainage modes for the cylindrical drain cover were determined, and the fundamental data for the stability of underwater local dry welding were provided.