<p>The high-temperature melting and crystallization differentiation technology is an effective method for enriching niobium resources in Bayan Obo, but the phase equilibrium relations are unclear during the melting and cooling crystallization process. In the present work, phase equilibria of the Nb₂O₅-CaO-MgO-SiO₂ system at 1250°C, 1300°C, and 1400°C in air were studied by the equilibrium experiments. The phase equilibrium relations were identified by electron-probe x-ray microanalysis and X-ray diffraction. The results revealed seven independent tetrahedral regions at 1250°C, including CaNb₂O₆-MgNb₂O₆-MgSiO₃-SiO₂ region, Ca₂Nb₂O₇-Ca₂MgSi₂O₇-CaMgSi₂O₆-Mg₂SiO₄ region, CaNb₂O₆-Ca₂Nb₂O₇-Mg₄Nb₂O₉-Mg₂SiO₄ region, CaNb₂O₆-CaMgSi₂O₆-MgSiO₃-SiO₂ region, CaNb₂O₆-CaSiO₃-CaMgSi₂O₆-SiO₂ region, Ca₂Nb₂O₇-Ca₂MgSi₂O₇-CaMgSi₂O₆-CaSiO₃ region, and Nb₂O₅-CaNb₂O₆-MgNb₂O₆-SiO₂ region. At 1300°C, two four-phase regions were identified as Liquid-Ca₂Nb₂O₇-Ca₂MgSi₂O₇-Mg₂SiO₄ and Liquid-CaNb₂O₆-MgSiO₃-SiO₂. Afterward, the liquidus surface of the phase relation Liquid-SiO₂ was plotted, and the Liquid-SiO₂ region was confirmed at 1400°C. The determined phase equilibrium relations can provide theoretical guidance for converting multiple niobium-bearing minerals into a single niobium-bearing phase by the high-temperature melting and crystallization differentiation technology.</p>