The low-reactive tundish flux is urgently required for the smelting of rare-earth steels. The effect of Ce2O3 content on the properties (melting point and viscosity) and structure of CaO–Al2O3–Ce2O3–MgO–SiO2 slag were analyzed in the present investigation. The research results indicated that the polymerized QAl4 and QAl3 units were modified to QAl2 units in aluminate, and the depolymerization of SiO4-tetrahedrons from QSi1 to QSi0 units with Ce2O3 addition increasing from 5 wt% to 20 wt%. In addition, when the Ce2O3 addition increased from 15 wt% to 20 wt%, the increasing area fraction in AlO6-octahedral units further led to a decrease in the degree of polymerization (DOP) in the melts. When the Ce2O3 addition increased from 5 wt% to 20 wt%, the melting temperature decreased from 1320°C to 1301°C. Additionally, the softening temperature and the fluidity temperature also decreased 20°C and 24°C, respectively. The viscosity decreased as the Ce2O3 addition increased from 5 wt% to 20 wt%, which was attributed to the decreased DOP and the increased superheat. A reasonable content for Ce2O3 should not be excess 10 wt% in the current slag.