Composite polymer electrolyte (CPE) films with high room-temperature ionic conductivity are urgently needed for the practical application of high-safety solid-state batteries (SSBs). Here, a flexible polymer–polymer CPE thin film reinforced by a three-dimensional (3D) bacterial cellulose (BC) framework derived from natural BC hydrogel was prepared via the in-situ photo-polymerization method. The BC film was utilized as the supporting matrix to ensure high flexibility and mechanical strength. The BC–CPE attained a high room-temperature ionic conductivity of 1.3×10−4 S cm−1. The Li|BC–CPE|Li symmetric cell manifested stable cycles of more than 1200 h. The LCO|BC–CPE|Li full cell attained an initial discharge specific capacity of 128.7 mAh g−1 with 82.6% discharge capacity retention after 150 cycles at 0.2 C under room temperature. The proposed polymer-polymer CPE configuration represents a promising route for manufacturing environmental SSBs, especially since cellulose biomaterials are abundant in nature.