While the Mg-Si system features only one thermodynamically stable compound, $\mathrm{M}{\mathrm{g}}_2\mathrm{Si}$, at ambient pressure, multiple new phases emerge under compression up to ∼100 GPa. Intriguingly, above 500 GPa, $\mathrm{M}{\mathrm{g}}_2\mathrm{Si}$ reemerges as the sole stable compound. In this work, we combine adaptive genetic algorithm crystal-structure prediction with machine learning interatomic potentials to systematically explore high-pressure stability of Mg-Si system at 200, 300, and 400 GPa. By constructing a pressure–composition phase diagram spanning 100 to 500 GPa, we identify several pressure-stabilized phases, $Cmmm \mathrm{M}{\mathrm{g}}_4\mathrm{S}{\mathrm{i}}_3$ and $R\overline{3}m \mathrm{M}{\mathrm{g}}_3\mathrm{Si}$, which exhibit unexpected stoichiometries and structural motifs. Through analysis of electronic structure, we demonstrate that the localization of electrons and their bonding character play a crucial role in the phase stability and structural evolution under high pressure. These findings suggest that Mg-Si compounds with diverse compositions could form in oxygen-deficient planetary interiors, and provide an extended structural database for exploring multicomponent systems containing Mg and Si under extreme conditions.

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• Crystal structure
• High-pressure studies
• Alloys
• Crystal structures
• First-principles calculations