Abstract: In this article, the autonomousrutile type carbon steel electrode was used for underwater wet welding. It focused on the effect of the welding current and thickness of electrode coating on phase and microstructure of slag during underwater wet welding. By macro-morphology analysis, it found that when the thickness of electrode coating was thin, the slag was thin and uneven coverage. Furthermore, welding current had an important effect on welding spatter. When the welding current was large, there was a large amount of splashing in the welding process. By the analysis of basicity coefficient and phase of slag, there was a little effect of thickness of electrode coating and welding current on the basicity coefficient of slag. The main composed phase of slag was FeTiO₃ and CaTiO₃, also contained a small amount of MnTiO₃, FeF₃ and Ca-Si-O phase. By the microstructure analysis of slag, with the decreasing of the electrode coating and increasing of the welding current, there was a large amount of dendritic structure was formed, the direction of the microstructure of slag was enhanced, which was advantageous to the detachability of slag.