<p>Sulfide-based all-solid-state lithium (Li) batteries with high safety, high energy density, and sustainable energy storage have promising applications. However, due to the incompatibility between Li anodes and sulfide solid electrolytes, which further restricts ion transport and uniform Li deposition, parasitic reactions at the interface are prone to occur, leading to Li dendrite formation. Herein, a high-diffusion-coefficient lithium-magnesium (Li-Mg) alloy was designed as a dendrite-free anode. When Li strips out, Li₃Mg₁₇ (α + β-LiMg) forms; upon Li plating back, a homogeneous and stable Li_0.9Mg_0.1 (β-LiMg) forms. The reversible phase transition between β-LiMg and α-LiMg alloys enables uniform, rapid lithium-ion transport. Density-functional theory (DFT) calculations indicate that the addition of Mg lowers the nucleation barrier of Li⁺ and reduces parasitic reactions, which promotes the uniform deposition of Li⁺ on the anode. The symmetric cell was assembled using a Li-Mg alloy anode and Li₆PS₅Cl as the solid-state electrolyte, achieving a critical current density of 0.9&#xa0;mA cm⁻² and stable cycling performance for more than 500&#xa0;h at 0.1&#xa0;mA cm⁻². Notably, to validate its practical application value, a complete battery assembled with LiNi₀.₈Co₀.₁Mn₀.₁O₂ coated with 0.6% LiNbO₃ as the optimal cathode material and a Li-Mg alloy as the anode demonstrated excellent rate performance and stable cycling performance. These findings provide valuable insights into selecting alloy anodes that inhibit dendrite growth.</p>