[en] Conventional liquid electrolytes based sodium metal batteries suffer from severe safety hazards owing to electrolyte leakage, inflammability and dendritic sodium deposition. Herein, we report a flame-retardant quasi-solid polymer electrolyte with poly(methyl vinyl ether-alt-maleic anhydride) (P(MVE-alt-MA)) as host, bacterial cellulose (BC) as reinforcement, and triethyl phosphate/vinylene carbonate/sodium perchlorate (TEP/VC/NaClO₄) as plasticizer for highly safe sodium metal batteries. The as-obtained quasi-solid polymer electrolyte exhibits superior flame retardancy (self-extinguish within 1 s), complete non-leakage property and wide electrochemical windows (4.4 V). More importantly, Na₃V₂(PO₄)₃/Na metal batteries using such polymer electrolyte delivers superior long-term cycling stability (84.4% capacity retention after 1000 cycles) which is significantly better than that (only 2% after 240 cycles) of liquid electrolyte. In addition, this flame-retardant quasi-solid polymer electrolyte provides favorable cycle performance (80.2% capacity retention after 70 cycles at 50 °C and 84.8% capacity retention after 50 cycles at −10 °C) for Na₃V₂(PO₄)₃/Na metal batteries. And this battery also displayed a normal charge/discharge property even at −15 °C. These fascinating cycle properties are mainly ascribed to the effective protective layers formed on Na₃V₂(PO₄)₃ cathode and sodium metal anode. More thorough investigation elucidates that such flame-retardant quasi-solid polymer electrolyte plays a multifunctional role in the advanced sodium metal batteries: (1) Being involved in the formation of a favorable cathode electrolyte interface (CEI) to inhibit the dissolution of vanadium and maintain the structure integrity of the Na₃V₂(PO₄)₃; (2) Participating in building a stable solid electrolyte interface (SEI) to suppress the growth of Na dendrites; (3) Integrating flame-retardance into polymer sodium batteries to enhance flame-resistance, eliminate electrolyte leakage, and thus improve safety of sodium batteries. Based on these results, we further assembled Na₃V₂(PO₄)₃/MoS₂ pouch cell which can withstand harsh conditions (bended or cut off a corner), confirming the obtained polymer electrolyte with superior non-leakage property. In all, these outstanding characteristics would endow this flame-retardant quasi-solid polymer electrolyte a very promising candidate for highly-safe sodium metal batteries. .