Solid-state batteries based on Li and nonflammable solid-state electrolytes (SSEs) have aroused the attention of numerous researchers because of their absolute safety and potentially high energy density. Most SSEs after coming into contact with Li are reduced, which leads to high interfacial charge-transfer impedance and dendrites formation. In this study, an “interlayer-Li pre-reduction strategy” was proposed to solve the above problem of reduction. An intermediate layer was introduced between solid electrolyte and Li, and it reacted with Li to produce a stable and ion-conductive interphase. Cubic garnet-type Nb-doped Li₇La₃Zr₂O₁₂ (Nb-LLZO) was selected as an example solid electrolyte since it is characterized by high ionic conductivity, feasible preparation under ambient conditions, as well as low cost. The high impedance arising from the reduction at the Nb-LLZO|Li interface has limited its application. In this paper, a nano-scale Li phosphorus oxynitride (LiPON) layer was deposited on the Nb-LLZO pellets through radio frequency (RF) magnetron sputtering, which pre-reacted with Li in-situ to produce a lithiophilic, electronically insulating, and ionic conductive interphase. The produced interphase significantly inhibited the reduction of Nb⁵⁺ against Li and the formation and propagation of Li dendrites. It is noteworthy that Li|LiPON|Nb-LLZO|LiPON|Li cells stably cycled for over 2,000 h without any short circuit. This study emphasizes and demonstrates the significance of the pre-conversion of modification layer between unstable SSE and Li metal to improve interfacial stability.