<p>Aqueous magnesium metal batteries (AMMBs) offer a compelling pathway toward safe, sustainable, and high-energy-density storage. However, their development is fundamentally constrained by severe water-induced parasitic reactions at the Mg-H₂O interface. Here, we identify the water-dominated Mg²⁺ solvation structure as the primary driver of this instability, thereby intensifying hydrogen evolution and Mg corrosion. Therefore, we design a solvation-regulation strategy by introducing 1,3-dioxolane (DOL) as a molecular modulator to reconstruct the Mg²⁺ solvation shell and disrupt the bulk hydrogen-bonding network, enabling reversible Mg plating/stripping. The Mg∥Mg symmetric cells could stably cycle over 1000 h and the assembled Mg∥CuHCF full cells achieve a higher operating voltage of ∼1.9 V with stable cycling performance of 1800 cycles (81% capacity retention). Furthermore, the practical pouch cell demonstrates stable operation and the decoupled Mg∥MnO₂ configuration that further delivers an operation potential of 2.7 V. This work provides fundamental insight into solvation-mediated interfacial chemistry and establishes a practical design rule for durable, high-voltage aqueous magnesium metal batteries.</p>