Abstract: Sulfide all-solid-state lithium metal batteries have received increasing attention owing to their high specific energy density and remarkable safety. However, serious interfacial problems still limit their further development. To solve the problem of instability of the interface between the solid-state electrolyte argyrodite (Li₆PS₅Cl) and lithium anode, strategies such as introducing an alloy cathode, introducing an intermediate interface layer, and directly modifying the electrolyte have been proposed; however, these methods are not suitable for practical applications. Notably, lithium lanthanum zirconium oxide (LLZTO) exhibits high lithium-ion conductivity and remarkable material stability, and silver (Ag) metal shows satisfactory lithium conductivity. Accordingly, a composite interface layer made of LLZTO and Ag was innovatively proposed to solve the lithium metal anode/Li₆PS₅Cl interface problem and increase the cycle stability of all-solid-state lithium batteries. We studied the effects of LLZTO–Ag composite interface layers with different combination manners, such as simply dispersed LLZTO–Ag composite, evenly dispersed and coated composite, and ball-milled composite, on the anode interface of Li₆PS₅Cl all-solid-state lithium metal batteries. The electrochemical performance of an optimized all-solid-state battery was also investigated. The results show that the surface of the LLZTO@Ag composite layer obtained by ball milling is relatively smoother and denser, which can effectively prevent lithium dendrite growth and battery short circuit. Compared with the simply dispersed LLZTO–Ag composite method and the evenly dispersed and coated composite method, the ball-milled composite layer anode method can be used to effectively reduce local lithium deposition current density and successfully solve the short circuit problem of the sulfide solid electrolyte. The first cycle efficiency of the LLZTO_pw@Ag_pw–Li_pl all-solid-state battery is 77.5%, and the discharge specific capacity is 187.3 mA·h·g⁻¹. After 100 cycles at 0.3C, the discharge specific capacity is still 125.5 mA·h·g⁻¹, and the capacity retention rate is 81.7%. Additionally, we investigated the electrochemical behavior of all-solid-state lithium metal batteries upon the introduction of the LLZTO–Ag composite interfacial layer by using the AC impedance (EIS) and constant-current intermittent titration technique. The LLZTO_pw@Ag_pw anode shows satisfactory cycle stability for lithium batteries. The impedance of the LLZTO_pw@Ag_pw–Li_pl all-solid-state battery exhibits periodic oscillations, indicating that lithium vacancies will be generated in the NCM811 crystal upon extraction of lithium ions, thereby increasing the conductivity of the lithium ions and reducing their migration resistance as well. The effect is most prominent when half of the lithium ions are extracted, but further extraction of lithium ions will lead to too many vacancies in the material, following which extraction of lithium ions will be impeded, thereby increasing the migration resistance of the lithium ions. The interfacial impedance on the cathode side considerably increased during long cycling, thus affecting the subsequent cycling performance, while the interface on the anode side remained essentially stable, highlighting the stabilizing effect of the LLZTO–Ag composite interfacial layer.