<p>The role of individual oxide components in vanadium slag is critical for optimizing the cleaner calcification roasting–acid leaching vanadium extraction process. The effect of MgO content (0 to 8.23 wt pct) on phase transformation and subsequent vanadium leaching behavior was systematically investigated using synthetic slags, combining FactSage thermodynamic calculations, XRD analysis, and SEM-EDS characterization. A non-monotonic dual role of MgO in phase reconstruction was revealed. Moderate MgO addition (1.91 to 3.74 wt pct) elevated the liquidus temperature and promoted the crystallization of V-bearing spinel and Mg-rich olivine, resulting in spinel grain growth and enhanced phase development. However, excessive MgO (&gt; 5.36 wt pct) induced a transition from a well-defined two-phase spinel–silicate intergrowth to a complex multiphase assemblage with refined spinel grains and reduced interfacial reactivity. During calcification roasting, this dual role manifested competitively. Moderate MgO facilitated the formation of acid-soluble calcium vanadates, whereas excessive MgO preferentially stabilized magnesium vanadates, thereby suppressing the desired calcification reactions. Consequently, vanadium leaching efficiency exhibited a non-monotonic dependence on MgO content, reaching a peak of 92.7 wt pct at 1.91-wt pct MgO and a CaO/V₂O₅ ratio of 0.7. Further MgO addition reduced efficiency and shifted the optimal CaO/V₂O₅ ratio to ~ 0.5 to 0.6. These findings elucidate the dual role of MgO in both phase reconstruction and roasting chemistry, providing a mechanistic basis for the composition-based optimization of the calcification roasting–acid leaching process.</p>