Abstract: By collecting and analyzing the underwater welding arc spectrum, the influence of the different water environment media such as clear water, boric acid solution, LiOH solution, and boric acid+LiOH mixture, on the composition and temperature of underwater welding arc plasma were studied. In addition, the composition and temperature of arc plasma during on-land welding and underwater welding were compared and analyzed. The results indicated that the effects of water media on underwater welding arc plasma are almost negligible. However, the H and O in arc plasma composition of underwater welding are significantly more than those in the on-land welding arc. Owing to the water environment, the arc temperature of underwater welding was lower than that of on-land welding arc.