The lack of reliable thermodynamic data for cerium-containing oxide systems has significantly hindered the efficient recoveryRecovery and utilization of rare earthRare earths elements (REEs)Rare earth elements from secondary resources. In this study, the phase equilibriumPhase equilibrium relationships in the Ce2O3-CaO-SiO2-5 wt% MgO quaternary system at 1773 K were systematically investigated through high-temperature equilibration and quenching experiments, followed by comprehensive characterization using XRD, SEM–EDS, and EPMA. The isothermal liquidus linesLiquidus line were precisely determined, and the stability regions of key phases, including SiO2, Ce2Si2O7, and CaO·2Ce2O3·3SiO2, along with their coexistence relationships, were unambiguously identified. Furthermore, a more accurate isothermal section diagram model was established through comparative analysis between thermodynamic calculations and experimental data. This study not only fills a critical gap in the thermodynamic database of rare earth slag systemsSlag system design but also provides fundamental insights for optimizing rare earthRare earths metallurgical slag design and achieving sustainable recoveryRecovery of cerium from industrial waste residues.