Magnesium metal batteries (MMBs) currently face challenges suffering from severe Mg metal passivation and extremely high overpotential in conventional electrolytes. Herein, a strategy of using a low‐cost deep eutectic solution (DES) is proposed to modify Mg anode with the monolithic and compact coating of a MgCl2‐Al‐MgCl2 sandwich structure, enabling the stable and reversible Mg plating–stripping behavior. An organic/nanocrystal hybrid interphase is in‐situ built through a facile Mg‐Al displacement reaction between aluminum‐chloro clusters and Mg in AlCl3/Et3NHCl solution, and it can effectively minimize the adverse interfacial passivation reaction and surface diffusion barrier, affording the high ion‐conduction and electronic insulation. This DES‐assisted method guarantees a highly reversible cycling of Mg metal anode (over 5000 h at 0.1 mA cm−2 and 400 h at 2.0 mAh cm−2) in Mg(TFSI)2/DME electrolyte with the improved interfacial kinetics and low overpotential. Even at a much higher current density of 1 mA cm−2, the overpotential only undergoes a slight increase from 0.2 V (at 0.1 mA cm−2) to 0.23 V. The corresponding full cells with CuS and phenanthraquinone cathodes deliver satisfactory cyclic performance. The DES modification strategy provides a new solution to the design of robust and conductive solid electrolyte interphase for achieving high‐voltage and durable MMBs.