Composite solid-state electrolytes (CSEs) with high ionic conductivity and low interfacial resistance have been pursued for next‐generation high energy density all solid-state lithium-ion batteries (ASSLIBs). Herein, we report a novel strategy of developing thin, flexible, and conductive CSEs by using lignin nanoparticles (LNPs) to regulate pore characteristics of cellulose nanofibril (CNF) film template. The CNF-LNP film was first prepared, then LNPs were removed, thanks to the excellent LNP solubility in γ-Valerolactone (GVL), resulting in the formation of uniform and porous cellulose nanofibril (CNF) film, which is then utilized for preparing Li7La3Zr2O12 (LLZO) membrane, with a superior morphological architecture (pore size, uniformity). Poly (ethylene oxide) (PEO) is then infiltrated into the membrane, producing thin, flexible, and conductive CSEs. Due to the controllable interconnected architecture of LLZO membrane from the LNP regulated CNF film template and its good compatibility with PEO, the as-prepared CSEs exhibit a high lithium ionic conductivity of 1.83 × 10−4 S cm−1 at ambient temperature. The continuous and uniform lithium transfer pathways, with excellent electrolyte/electrode interface contact contribute to long-term cycling stability of symmetric lithium batteries. Hence, the assembled ASSLIBs from the as-prepared CSEs show high discharge specific capacities of 157 and 159.5 mAh g−1 with LiFePO4 and LiNi0.5Mn0.3Co0.2O2 respectively, with attractive cycling stabilities and capacity retention at ambient temperature.