In this work, an “upright” cellulose layered template, which can be precisely controlled by the fiber orientation and thickness of anisotropic cellulose film, was first developed to prepare vertically aligned Li₇La₃Zr₂O₁₂ (LLZO) ceramic membrane with low tortuosity and uniform porosity. The flexible composite solid‐state electrolytes (CSEs) were synthesized by incorporating poly(ethylene oxide) (PEO) polymer into LLZO architecture, and exhibited distinguished mechanical strength, elevated ionic conductivity (2.1 × 10⁻⁴ S cm⁻¹) and excellent interface compatibility with lithium anodes. The assembled LiFePO₄/lithium full cells assembled with as‐obtained CSEs can reach large discharge capacities of 172.3 mAh g⁻¹ with coulombic efficiencies of above 99% and capacity retention of 93.1% even after 200 cycles. These results clearly demonstrate that the sophisticated design of vertical aligned cellulose layered template with fine‐scale size and exquisite architecture is a promising and practical approach in fabricating flexible LLZO@PEO‐LiTFSI CSEs with high‐safety and high‐performance. image