Abstract: In this paper, the autonomous rutile type carbon steel electrode was used for underwater wet welding. It focused on the effect of the addition amount of TiO₂ in the coated on the morphology and microstructure of slag. By composition and phase analysis of slag, the main phase of slag was Fe₂MnTi₃O₁₀, and it also contained a small amount of Ca₂SiO₄ and free oxides. By morphology and microstructure analysis of slag, it found that when the addition amount of TiO₂ was larger, the slag was compact. Furthermore, the gray stripe phase in the microstructure of slag was bulky, which was advantageous to the detachability of slag. With the decrease of addition amount of TiO₂ and increase of addition amount of CaCO₃, the slag became loose and viscous slag occurred. Meanwhile, the gray stripe phase in the microstructure changed thin, which was not advantageous to the detachability of slag.