<p>MgO has been shown to facilitate the precipitation of MgO-rich crystalline phases within the MgO–CaO–Al₂O₃–SiO₂ (MCAS) glassy inclusion system, which possesses a high liquidus temperature and a significant Young’s modulus. The underlying linkage between the structural evolution and the crystallization characteristics of the MCAS system was systematically investigated using molecular dynamics simulation and thermodynamic calculation. The results revealed that Mg²⁺ ions played a dual role, constructing networks through the formation of tricluster oxygens while consuming bridging oxygens (BOs) in a mechanism similar to Ca²⁺ ions. However, despite this dual role, the network connectivity was still decreased with the increase in MgO/(MgO + Al₂O₃) (M/(M + A)) and CaO/(CaO + SiO₂) (C/(C + S)) ratios, primarily due to the reduction in BOs. This microscopic structural evolution resulted in a reduction in viscosity and an enhancement of crystallization ability. Furthermore, the remarkable diffusion capability of Mg²⁺ ions, coupled with the increased proportion of 6-coordinated Mg²⁺ ions, unveiled the mechanism underlying the precipitation of MgSiO₃ and Mg₂SiO₄ crystals, which exhibited high Young’s moduli of 165.23 and 196.67&#xa0;GPa, respectively. To prevent the precipitation of MgO-rich crystalline phases, it was crucial to maintain the M/(M + A) ratio below 0.42 and the C/(C + S) ratio below 0.16 within the MCAS system.</p>