The low ionic conductivities, poor high-voltage stabilities, and lithium dendrite formation of polymer solid electrolytes preclude their use in all-solid-state lithium metal batteries (ASSLMBs). This work provides a simple and scalable technique for constructing fast ion conductor nanofibers (FICNFs) and poly-m-phenyleneisophthalamide (PMIA) nanofibers synergistically enhanced polyethylene oxide (PEO)-based composite solid electrolytes (CSEs) for ASSLMBs. The FICNFs, which were mainly composed of high loadings of ZrO2 or Li6.4La3Zr1.4Ta0.6O12 nanoparticles, had a percolated ceramic phase inside the nanofibers, while the exposed nanoparticles formed continuous organic–inorganic interfaces with the PEO matrix to enable Li+ transport. The interfacial transport rate between ZrO2 and PEO was calculated as 4.78 × 10–5 cm2 s-1 with ab initio molecular dynamics (AIMD) simulations. Besides, the PMIA nanofibers provided strong skeletal support for the CSEs, ensuring excellent mechanical strength and safety for thin CSEs even at high temperatures. More importantly, the amide groups in PMIA provided abundant hydrogen bonds with TFSI-, which lowered the lowest unoccupied molecular orbital (LUMO) level of lithium salts, thus promoting the generation of lithium fluoride-rich solid electrolyte interphase. Consequently, the modified CSEs exhibited satisfactory ionic conductivities (5.38 × 10–4 S cm-1 at 50 °C) and notable Li dendrite suppression (> 1500 h at 0.3 mAh cm-2). The assembled LiFePO4||Li full cells display ultra-long cycles (> 2000 cycles) at 50 °C and 40 °C. More strikingly, the LiNi0.8Mn0.1Co0.1O2 (NMC811)||Li cell also can stably run for 500 cycles, and the LiFePO4||Li flexible pouch cells also cycled normally, demonstrating tremendous potential for practical application.