Amid the continuing rise in global demand for magnesium metal, the considerable reserves of magnesium within magnesium slag remain insufficiently recovered, resulting in notable resource wastage and increased vulnerability to supply chain instability. Current mainstream approaches for the comprehensive utilization of magnesium slag have yet to demonstrate feasibility for large-scale industrial deployment. In this work, a novel synergistic activation approach—vacuum carbothermal reduction coupled with CaF₂ catalysis—is proposed. This method enables precise regulation of key parameters within the reduction system to harness the full potential of the intrinsic Ca₂SiO₄ phase in magnesium slag. Under high-temperature conditions, Ca₂SiO₄ interacts in situ with added CaF₂ flux to generate a low-melting-point eutectic system, substantially reducing the reaction’s activation energy and accelerating mass transfer. These combined effects promote the efficient reduction of MgO and the highly selective liberation of Mg(g). Experiments show that this technology achieves a MgO reduction rate ≥90 % in magnesium slag, with a direct collection efficiency rate ≥85.14 %, and the purity of regenerated crystallized magnesium stabilizes at ≥88.18 %. Extending the holding time has been proven to have a dual optimization effect: first, by enhancing the catalytic efficiency of CaF₂, the MgO reduction efficiency is improved by 5.51 % (when the holding time is extended by 1 h); second, it promotes uniform nucleation and equiaxed crystal growth of Mg(g) at only further increasing the purity of crystallized magnesium by +5.24 %, but it also significantly enhances its grain integrity and microstructural uniformity. This regenerated magnesium crystal, characterized by high purity and low defect density, provides an excellent microstructural foundation for subsequent plastic forming or service applications.