Calcium ferrite (CF) is the adhesive phase formed in high-basicity sinter. The wettability of CF with other solid phases plays an important role in assimilation process. In this study, an improved sessile drop technique was used to explore the wettability of a CF series slag, in which constant contents of Al₂O₃, MgO, SiO₂, and TiO₂ were added, with solid Al₂O₃ substrate at 1250°C. The interfacial structure and spreading mechanisms were also discussed. The dissolution of Al₂O₃ into the slag was found to be the driving force of the wetting process. The CF series slag could rapidly spread after melting until a low apparent contact angle of approximately 10°–20° was attained. The spreading time positively correlated with the dissolution amount. The addition of MgO, SiO₂, and TiO₂ improved the wettability of CF on Al₂O₃ and decreased the surface tension of CF melt. After cooling, a three-layer structure was formed in the slag phase.