To improve the dephosphorization capacity of CaO–Al2O3–SiO2–MnO based ferromanganese slag, this work attempts to introduce Ce2O3 as a strong phosphorus fixative into the slag, and the influence mechanism of Ce2O3 on the dephosphorization process is investigated by thermodynamic analysis, structural analysis, melting temperature, and viscosity tests. The results show that after adding 20 mass% Ce2O3 into the slag, Ce2O3 can release the O2− and promote the P-O0 bond to P-O− bond transformation, thus improving the phosphorus fixation capacity of melted slag. Also, the phosphorus enrichment phases are nCa2SiO3–Ca3P2O8 solid solution and CePO4 phase in Ce2O3-containing slow cooling slag. Furthermore, the melting temperature of slag with 0–20 mass% Ce2O3 addition is always about 1623 K, but increases obviously when the addition of Ce2O3 exceeds 20 mass%. Also, adding appropriate Ce2O3 content into the CaO–Al2O3–SiO2–MnO based slag can decrease the viscosity of slag, and the viscosity can remain relatively low with adding 15 mass%–25 mass% Ce2O3. In conclusion, adding appropriate Ce2O3 content into the CaO–Al2O3–SiO2–MnO based slag can improve the phosphorus fixation capacity of slag and ameliorate the fluidity of slag, and consequently, it is conducive to remove phosphorus from ferromanganese. Finally, the high-temperature ferromanganese dephosphorization experiments were carried out, and the results show that after adding 20 mass% Ce2O3 into the slag, the lowest [P] content decreases from 0.31% to 0.25%, and the dephosphorization rate increases from 31.1% to 45.6%. The above results not only verify that the feasibility of Ce2O3 as a strong phosphorus fixative used in the ferromanganese dephosphorization slag, but also provide the theoretical guidance for optimizing the composition of Ce2O3-containing dephosphorization slag.