<p>The beneficial effects of ultrasonic frequency pulse current (UFPC)–assisted underwater wet flux-cored arc welding (UWFCAW) on the welded joint performance have been experimentally confirmed. However, the mechanism by which UFPC parameters influence the temperature and flow fields of the arc-bubble system remains unclear, limiting further application prospects of this technology. To overcome these limitations, this study developed a multiphysics-coupled model to investigate heat and mass transfer processes in the arc-bubble system under varying UFPC parameters. By analyzing the UFPC waveform via Fourier series expansion and applying electromagnetic field theory, a novel approach was developed to elucidate the ultrasonic wave generation mechanism induced by the arc. This mechanism was subsequently modeled by incorporating the resulting ultrasonic radiation force as a time-varying source term in the momentum conservation equation. Simulation results indicated that increasing peak current and UFPC frequency significantly enhanced momentum output and pressure concentration in the arc core region, promoting stable bubble evolution. This synergistic effect improved arc thermal characteristics, reduced temperature and pressure fluctuations, and ensured continuous energy delivery to the weld pool. Moreover, higher peak currents expanded the high-temperature plasma region, while elevated UFPC frequencies enhanced arc stability and thermal symmetry. The combination of high peak current and UFPC frequency further improved momentum transport and flow stability by increasing plasma velocity and vortex strength. Experimental observations of bubble morphology, evolution period, and volume closely matched the simulation results, confirming the model’s accuracy. These findings provide valuable theoretical support for optimizing process parameters to improve weld quality and stability in UWFCAW.</p>