A cross-linked porous polymer membrane using vinyl-functionalized aluminum oxide (Al2O3) nanoparticles was prepared and used as a separator for lithium-ion batteries (LIBs). Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was first blended with a mixture solution containing poly(ethylene glycol) methyl ether methacrylate (PEGMEMA), vinyl-functionalized Al2O3 nanoparticles (VTMO@Al2O3) as the cross-linker and poly(vinyl pyrrolidone) (PVP) as the pore-forming agent. Subsequently, the membrane was obtained by free radical copolymerization of the above-mentioned mixture and after-treatment. The preparation and properties of the membranes were investigated. It has been found that, under the action of the pore-forming agent of PVP, the obtained membrane (PMAv) had high ionic conductivity (1.37 mS cm−1) at ambient temperature. On account of the existence of the Al2O3 as cross-linking points, the PMAv membrane showed good mechanical strength of 30.4 MPa and excellent thermal stability at 180 °C. Moreover, comparing with the system without PVP or Al2O3 samples, the cell with the PMAv membrane showed the best discharge capacity and most stable capacity retention and cycle performance, indicating that the PMAv membrane is a good polymer separator for LIBs.