The ever-increasing demand for safe and high-energy-density batteries urges the exploration of ultrathin, lightweight solid electrolytes with high ionic conductivity. Solid polymer electrolytes (SPEs) with high flexibility, reduced interfacial resistance and excellent processability have been attracting significant attentions. However, reducing the thickness of SPEs to be comparable with that of commercial separators increases the risk of short-circuiting. Herein, an ultrathin (≈7 μm), flexible and mechanical robust SPE was constructed from a rationally designed multi-functional polymer network, i.e., poly[2,2,2-trifluoroethyl methacrylate-r-(2-ethylhexyl acrylate)-r-methyl methacrylate-r-1,4-bis(acryloyloxy)butane] (PTEM) and porous polyethylene (PE). The resultant PTEM@PE electrolyte possesses a high tensile strength of 128.0 MPa with extensibility up to 34.8%, which could effectively prevent short-circuiting and minimize the interfacial resistance of cells. The obtained all-solid-state Li|PTEM@PE|LiFePO4 cell exhibited stable cycling performance over 1500 cycles at 0.5 C with a capacity retention of 74.4%. With high-voltage NCM811 as the cathode, the cell fabricated with PTEM@PE showed a remarkable capacity retention of 84.2% over 500 cycles. Even with the high-mass loading (≈3 mA h cm-2) NCM811 cathode, the cell could be operated at ambient temperature, demonstrating superior ion-migration kinetics. The current design provides a promising strategy to develop ultrathin and multifunctional solid electrolytes for safe, long-cycling and high-energy-density all-solid-state batteries.