Flexible batteries such as scalable fiber-shaped lithium-ion batteries (FLIBs) hold great potential for powering wearable electronics due to their excellent electrochemical performance, flexibility, and weavability. However, the use of organic liquid electrolytes raises serious safety concerns, including leakage and combustion hazards. In this study, we develop fire-resistant lithium cobalt oxide/graphite FLIBs by employing an in situ polymerized gel polymer electrolyte (GPE) incorporating 1,3,3,5,5-pentafluoro-1-ethoxy-cyclotriphosphazene (PFPN) as a flame retardant and triethylene glycol dimethacrylate (TEGDMA) as a crosslinker. This in situ polymerization of GPE is well-suited for continuous FLIB production and enhances the electrolyte/electrode interface. The resulting GPE eliminates the inherent flammability of liquid electrolytes with zero self-extinguishing time, attributed to a dual flame-retardant mechanism: gas-phase free radical scavenging and condensed-phase carbon formation. Notably, the FLIBs using the flame-retardant GPEs demonstrate no thermal runaway and maintain nonflammability under various abusive conditions, including mechanical abuse (cutting or collision), electrical abuse (overcharging or overdischarging), and thermal abuse (overheating or fire hazards). Additionally, these FLIBs achieve excellent cycling stability over 500 cycles, and retain 99.4 % of capacity after 10,000 bending cycles, highlighting their outstanding durability. This work presents an effective and straightforward approach to greatly enhance the safety and practicality of FLIBs for wearable applications.