The lack of phase diagrams and other related thermodynamic information for the silicate slag system with additions Nb and REE seriously restrict the comprehensive utilization of REE-Nb-Fe ore deposit resources in China. In this study, the equilibrium phase relations in the CaO–SiO2–Nb2O5–La2O3 quarternary system at 1273 K and 1473 K were investigated experimentally using the high-temperature equilibrium experiment followed by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). Subsolidus phase relations in the CaO–SiO2–Nb2O5–La2O3 quarternary system and La2O3–SiO2–Nb2O5 ternary system were determined and presented in the form of independent tetrahedron regions and triangle regions, respectively, according to the Gibbs Phase Rule, the isothermal section for 5 wt%La2O3 and 10 wt%La2O3 at 1273 K were also constructed respectively.