Na superionic conductor (NASICON)-type Li_1.5Al_0.5Ge_0.5P₃O₁₂ (LAGP) solid state electrolytes (SSEs) have attracted significant interests thanks to the prominent ionic conductivity (> 10^–4 S·cm^–1) at room temperature and superb stability in air. Unfortunately, its application has been hindered by the lithium dendrites and the intrinsic interfacial instability of LAGP towards metallic Li, etc. Herein, by magnetron sputtering (MS), an ultrathin Al film is deposited on the surface of the LAGP pellet (Al-LAGP). By in-situ alloying reaction, the spontaneously formed LiAl buffer layer inhibits the side reaction between LAGP SSEs and Li metal, and induces the uniform distribution of interfacial electric field as well. Density functional theory (DFT) calculations demonstrate that the LiAl alloy surface promotes the diffusion of lithium atoms due to the lower energy barrier, thereby inhibiting the formation of lithium dendrites. Consequently, the Li/Al-LAGP-Al/Li symmetric cells show a low resistance of 210 Ω and a durable lifespan over 1,200 h at a high current density of 0.1 mA·cm^–2. Assembled all solid state lithium metal batteries (ASSLMBs) with LiFePO₄ (LFP) cathode significantly improve cycle stability and rate performance, proving a promising stabilization strategy towards the NASIOCN type electrolyte/anode interface in solid state Li metal batteries.