Abstract: Objective To address issues such as arc instability, frequent arc extinction and coarse weld microstructure in underwater wet welding, an ultrasonic-assisted powder underwater wet welding test platform was established. The study comparatively investigated the influence patterns of conductive graphite and dielectric silicon dioxide powders on arc characteristics and weld quality. Methods Under uniform welding parameters and ultrasonic conditions, two particle size ranges and three mass concentrations (0.5 g/L, 1 g/L, 2 g/L) were selected. By regulating powder type, particle size, particle concentration, current and voltage waveforms were recorded, and probability density distribution was calculated, arc extinction times and occurrence frequency was statistically analysed. Combined with macromorphology and microstructure analysis of welds, this enabled to evaluate systematically synergistic effects between powders and ultrasonic cavitation. Results The results showed that fine-grained graphite powder established a multi-point conductive network within underwater arc channel, reducing arc fluctuations and probability of short circuits and arc breaks. Through heterogeneous nucleation, carbon diffusion and deoxidation, it promoted the formation of fine-grained ferrite and lower bainite, significantly enhancing joint strength and toughness. Silicon dioxide powder generally diminished arc stability under most conditions, exhibiting arc extinction inhibition only when specific particle size and concentration were matched. However, it lacked of metallurgical activity and nucleation capability which readily induced coarse brittle structures such as blocky ferrite, upper bainite and martensite within welds. Conclusion The intrinsic electrical properties of powders, along with matching relationship between particle size and concentration that exerted a critical regulatory influence on underwater arc combustion behaviour and microstructure properties of welds. Graphite powder demonstrated superior advantages in enhancing the comprehensive performance of welded joints, thereby providing theoretical support for optimising ultrasonic-assisted powder underwater wet welding processes.