Composite solid electrolytes hold the promise of merging complementary merits of solid polymer electrolytes and ceramic fillers to achieve solid batteries with comprehensive performance. Especially, three-dimensional inorganic electrolyte frameworks, such as Li₇La₃Zr₂O₁₂, with fast and continuous lithium ion migration channels demonstrate great promise in composite solid electrolytes. Nevertheless, brittle ceramic conductor skeletons are incapable of providing sufficient mechanical adaptability, which restricts their practical application. Herein, a flexible, ion-conducting network which integrates Li₇La₃Zr₂O₁₂ nanoparticles in polyacrylonitrile nanofibers is fabricated through electrospinning method. Subsequently, a composite electrolyte with three-dimensional continuous structure is achieved via in situ polymerizing of 1,3-dioxolane within the ionic conduction framework. The highly conductive Li₇La₃Zr₂O₁₂ reinforced polymer nanofibers are not only available to promote transportation of lithium ion, but also provide structural flexibility and mechanical robustness for composite electrolyte. Accordingly, the obtained composite electrolyte combines enhanced room temperature ionic conductivity (4.38 × 10⁻⁴ S·cm⁻¹) with structural flexibility and mechanical robustness, supported by exceptional interfacial compatibility with lithium metal, enabling ultra-stable lithium symmetric battery operation (3000 h at 0.1 mA·cm⁻²). Furthermore, as-prepared LiFePO₄ and LiCoO₂/lithium solid-state batteries deliver high capacity retention of 96% after 350 cycles and capacity retention of 82% after 600 cycles at room temperature. This work provides a new avenue in design of advancing composite solid electrolytes.