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CeO₂ can be applied to refining slag to minimize the size of inclusions, speed up the deoxidization process, and adsorb Al₂O₃ inclusions. The impact through which CeO₂ content affects slag’s melting efficiency is still uncertain. The thermal analyzer was used to measure the thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) curves of the slag melting process. According to the study results, with the increase in CeO₂ content, the melting temperature of slag decreased first and then increased. The slag’s melting point fell from 1364 °C to 1324 °C and then rose to 1503 °C. XRD and SEM were used to analyze the CaO-Al₂O₃-SiO₂-MgO-CeO₂ slag’s microstructure. The mineral-phase structure of CeO₂-containing refining slag was primarily composed of Ca₂SiO₄ and 3CaO·Al₂O₃, MgO, SiO₂, CaO·Al₂O₃, and Ca₈Ce₆Al₆O₂₆. The proportion of 3CaO·Al₂O₃, CaO·Al₂O₃, and Ca₂SiO₄ decreased as the rare-earth-oxide content increased, while the proportion of Ca₈Ce₆Al₆O₂₆ increased. FactSage was used to estimate the equilibrium-phase compositions of slags with various compositions, and a model for predicting melting points was carried out by a linear regression model. Results were obtained through the analysis of equilibrium-phase composition and crystal structure transformation. The main reasons for the melting point decrease were the change of degree of polymerization and the decrease in contents and complete melting temperature of high-melting-point Ca₃Al₂O₆ and Ca₂SiO₄ compounds. The latter increase in melting point was due to the number of Ca₈Ce₆Al₆O₂₆ compounds and precipitation temperature increases and the complexity of the structural-network increases.