Abstract: Aiming at the ultra-high dynamic characteristics and precise control requirements of welding power supply in the extremely complex underwater environment, the main circuit of fast-frequency pulsed welding power supply based on SiC module is designed, an all-digital control system is developed, and the local dry underwater fast-frequency pulsed MIG (LDU-FFPMIG) welding power supply is exploited. The welding power supply has a rated output current of 400 A, which can accurately output fast-frequency pulsed current waveforms with the fast-frequency frequency of 0~30 kHz and the fast-frequency current amplitude of 200 A. A new LDU-FFPMIG process is proposed, and the fast-frequency pulsed is applied to perform 304 stainless steel local dry underwater MIG welding, the influence of fast-frequency pulsed on local dry underwater pulsed MIG welding is revealed. The results show that the self-developed LDU-FFPMIG welding power supply achieves stable local dry underwater MIG welding and good weld formation. The introduction of fast-frequency pulsed current significantly improves arc energy density and arc stability, reduces the melting width (B), increases the penetration depth (H), reduces the weld forming coefficient (B/H) by about 30%, and refines the grain.