Substituting liquid electrolytes with solid electrolytes is considered as an important strategy to solve the problem of flammability and explosion for traditional lithium-ion batteries (LIB). However, neither inorganic solid electrolytes (ISE) nor solid polymer electrolytes (SPE) alone can meet the operating requirements for room-temperature (RT) all-solid-state lithium metal batteries (ASSLMB). Here, we report a three-dimensional (3D) nanofiber framework reinforced polyethylene oxide (PEO)-based composite polymer electrolytes (CPE) through constructing a nanofiber framework combining polyacrylonitrile (PAN) and fast Li-ion conductor Li 0.33 La 0.557 TiO 3 (LLTO) framework by electrospinning method. Meanwhile, the PEO electrolyte filled in the pores of the PAN/LLTO nanofiber framework can effectively isolate the direct contact between the chemically active Ti 4+ in LLTO with lithium metal, thereby avoiding the occurrence of interfacial reactions. Enhanced electrochemical stability makes a wide electrochemical window up to 4.8 V with an ionic conductivity of about 9.87 × 10 –5 S·cm −1 at RT. Benefiting from the excellent lithium dendrite growth inhibition ability of 3D PAN/LLTO nanofiber framework, especially when the mass of LLTO reaches twice that of the PAN, Li/Li symmetric cell could cycle stably for 1000 h without a short circuit. In addition, under 30 °C, the LiFePO 4 /Li ASSLMB using such CPE delivers large capacities of 156.2 and 140 mAh·g −1 at 0.2C and 0.5C, respectively. These results provide a new insight for the development of the next generation of safe, high-performance ASSLMBs. Graphical Abstract