A lithium metal anode is regarded as the most promising anode material for the next generation of high-energy density batteries because of its high specific capacity and low reduction potential. However, dendritic deposition and severe side reactions in continuous Li plating/stripping inevitably hinder the practical application of Li metal batteries. A solid polymer electrolyte protective layer with synergistic Li₃PO₄/polyvinyl alcohol (PVA) features is in situ constructed on a lithium metal anode to obtain a stable interface during charge/discharge cycles. The protective layer can adapt to volume changes and inhibit lithium dendrites. The in situ reaction guaranteed the uniformity of ion transport and a tight interface between the protective layer and the lithium metal, so that the lithium deposition behavior was effectively regulated. The PP-Li anode presented a stable Li plating/stripping for 1000 h in a symmetrical cell system and exhibited an enhanced performance of the lithium titanium oxide cell. The in situ Li₃PO₄/PVA solid polymer electrolyte protective layer provided a promising strategy to tackle the challenges raised by the intrinsic properties of the lithium metal anode.