An organic/inorganic hybrid nanocomposite, poly(methyl methacrylate) grafted titanium dioxide (PMMA-g-TiO2), is synthesized by atom transfer radical polymerization (ATRP). The hybrid nanocomposites are incorporated into poly(vinylidene fluoride) (PVdF) membranes during the electrospinning process to fabricate a composite nanofibrous membrane. Then the resultant fibrous polymer electrolyte is obtained by immersing the composite membrane into liquid electrolyte. FTIR, TEM, TGA, GPC, SEM, XRD, and DSC are used to characterize the structure, morphology and thermal properties of PMMA-g-TiO2 hybrid nanocomposite and the composite nanofibrous membrane. The composite nanofibrous membrane is proven to be a good absorbent for the liquid electrolyte, and it exhibits a high electrolyte uptake and a high electrolyte retention ratio. The incorporation of PMMA-g-TiO2 into the nanofibrous membrane inhibits the crystallization of PVdF during the solidification process and improves the ionic conductivity of the fibrous polymer electrolyte from 2.51 × 10−3 to 2.95 × 10−3 S cm−1 at 20 °C. The electrochemical stability window of the polymer electrolyte is also enhanced due to the presence of PMMA-g-TiO2.