(NH₄)₂SO₄ roasting–water leaching process was studied as a method for improving the utilization efficiency of magnesite. Transformation of the mineral phases during roasting was evaluated by X-ray diffraction (XRD) analysis, combined with thermogravimetry-differential thermal analysis (TG-DTA). The data were compared with the phase transformation of MgO during roasting using (NH₄)₂SO₄. The experimental results demonstrated that at a (NH₄)₂SO₄/ magnesite (MgO in ore) molar ratio of 1.6:1, roasting temperature of 475 °C, and roasting time of 2 h using a particle size of ~74 μm, the magnesium extraction rate reached a maximum of 98.7%. The phase transformation during roasting could be summarized as follows: MgCO₃ was transformed into (NH₄)₂Mg₂(SO₄)₃, and then decomposed into MgSO₄. The reaction followed the mixed control mechanism involving chemical reaction control and diffusion control, with an apparent activation energy of 33.02 kJ/mol.