Polymer electrolytes are recognized as the optimal solution to resolve the bottlenecks of liquid electrolytes such as safety hazards, decomposition and sodium dendrite problems. Herein, a novel multifunctional monomer which combines nonflammable spirocyclic pentaerythritol diphosphates and the easily polymerizable acrylates are designed and prepared, and the corresponding cross-linked gel copolymer electrolytes are synthesized by in situ polymerization with ethyl acrylate and trifluoroethyl methacrylate. The gel polymer electrolytes have good thermal stability, good interfacial compatibility, a high ionic conductivity of 3.26 × 10⁻³ S cm⁻¹, a high ion transfer number of 0.604, and excellent electrochemical stability of up to 4.9 V (vs. Na⁺/Na), and thus could be used as possible less flammable electrolytes. The Na₃V₂(PO₄)₃ cells using gel polymer electrolytes deliver better long term cycle stability than the corresponding liquid electrolyte, which retain 81.4% and 69.5% capacity after 1600 cycles, respectively. Moreover, the gel polymer electrolyte based Na₃V₂(PO₄)₃ cell exhibits much higher capacity and superior cycle stability compared to the liquid cell when running at an elevated temperature of 60 °C. Furthermore, within the operating temperature, the gel polymer electrolyte cell has better reversibility of temperature adjustment. In addition, gel polymer electrolytes also exhibit good cycle performance with organic cathodes, holding great promise for the production of safer phosphate-containing cell systems.