To improve the safety concerns of lithium-ion batteries, a flexible scalable composite electrolyte film (FSCEF) was fabricated based on a fiber-shaped ceramic and polymer support. Ceramic fibers of Li1.3Ti1.7Al0.3(PO4)3 (LATP) were prepared by sintering the precursor-coated sacrificial template and then infiltrated with a polyethylene oxide (PEO) polymer to obtain the FSCEF. The LATP fibers induced continuous Li+ ion channels, allowing the FSCEF to show an ionic conductivity exceeding 10−4 S cm−1 at 60 °C. The synergistic action of the ceramic frameworks and supportive PEO resulted in enhanced mechanical flexibility. Furthermore, the possibility of using a FSCEF in all-solid-state batteries was confirmed by conducting electrochemical performance tests on a Li/FSCEF/LCO (LiCoO2) cell. We expect that the herein reported findings will contribute to the synthesis of thin and flexible solid-state electrolyte films with manufacturing scalability for promising high-voltage all-solid-state batteries.