Polyvinylidene fluoride (PVDF)-based solid polymer electrolytes (SPEs) show great potential for solid-state lithium metal batteries (SSLMBs) due to their high ionic conductivity, wide electrochemical window, and cost-effectiveness. However, they suffer from low room-temperature ionic conductivity and poor electrode–electrolyte interfacial compatibility, limiting their practical application. Herein, an organic–inorganic synergistic modification strategy is proposed that hexadecyltrimethoxysilane (HDTMS)-modified nano-silica (Mod-SiO₂) is incorporated into a PVDF/polyacrylonitrile (PAN)/lithium salts blend to fabricate a composite solid electrolyte (PVDF/PAN/lithium salt/Mod-SiO₂ (PPLS) composite system). Specifically, PAN facilitates PVDF molecular chains’ motion to enhance segmental mobility, while uniformly dispersed Mod-SiO₂ constructs continuous three-dimensional (3D) Li⁺ transport channels. This synergistic “ion-transport highway” effect boosts Li⁺ mobility and optimizes dispersion uniformity, ionic conductivity, and interfacial stability. At room temperature, the PPLS exhibits an ionic conductivity of 4.83 × 10⁻⁴ S·cm⁻¹, a Li⁺ transference number of 0.7, and an extended electrochemical window of 5.1 V. LiFePO₄|PPLS|Li batteries retain 80.9% capacity retention after 1000 cycles, with a Coulombic efficiency of 99.3%. This work provides valuable insights for the rational design and optimization of PVDF-based SPEs and advances the practical development of high-safety SSLMBs.