Gel polymer electrolyte (GPE) is considered to be a promising electrolyte to realize flexible and safe lithium-based batteries. However, the potential is severely hindered by low ionic conductivity under room temperature, undesirable mechanical property and unstable thermal performance. Herein, these puzzling issues can be effectively settled by constructing a 3D cross-linked network GPE through in-situ polymerization methyl methacrylate (MMA) and ethoxylated trimethylolpropane triacrylate (ETPTA) monomers on electrospun polyacrylonitrile (PAN) nanofibers. The shape conformable GPE presents a high ionic conductivity (0.33 mS cm−1 at room temperature), extended cycling stability even at high temperature and excellent ability to restrain the growth of the lithium dendrite. Consequently, Li|GPE|LiFePO4 cell shows 134.8 mAh g−1 initial discharge capacity and maintains 78.6% capacity retention after 600 cycles at 2 C under room temperature. More significantly, 112.3 mAh g−1 discharge capacity is still achieved at 2 C and under 50 °C after 300 cycles, which is far more superior than liquid electrolyte with PAN substrate separator. Notably, pouch cells remain working under bending and cutting conditions showing excellent flexibility. The eminent performances provide a promising and alternative way in electrolyte fabrication for lithium-based batteries.