Abstract: The effects of TiO₂ and Al₂O₃ on the viscosity, melting temperature, and surface tension of CaO-SiO₂-CaF₂ base welding slag were studied. The phase and structure characteristics of quaternary slag were analyzed using XRD and Raman spectroscopy. The results show that increasing the TiO₂ content can significantly decrease the viscosity, melting temperature, and surface tension of the welding slag, thereby improving the fluidity. With increasing Al₂O₃ content, the viscosity, melting temperature, and surface tension of the welding slag increase slightly at first and then decrease markedly. The viscosity of CaO-SiO₂-CaF₂-TiO₂ and CaO-SiO₂-CaF₂-Al₂O₃ quaternary welding slag is below 0.2 Pa·s after complete melting, with no significant effect from increasing temperature. Below the melting temperature, the slag solidifies rapidly with decreasing temperature, exhibiting obvious short-slag characteristics. XRD and Raman spectroscopy results show that the quaternary welding slag mainly contains Ca₄Si₂O₇F₂, a high melting point phase with a silicate network structure. Adding TiO₂ and Al₂O₃ can transform the silicate network structure into SiO₄⁴⁻ with a simple tetrahedral structure. CaTiO₃ forms in slag containing TiO₂. CaF₂ phase and a small amount of Ca₂Al(AlSiO₇) form in slag containing Al₂O₃. Some sensitive ranges of TiO₂ and Al₂O₃ content that greatly influence the melting characteristics of slag were found. This has certain reference significance for the formulation design of high-strength welding materials.