Sodium‐ion batteries (SIBs) based on solid‐state electrolytes (SSEs), although safe for high temperatures, are less capable of transferring ions at ambient temperatures, let alone at low temperatures. This work offers a simple and scalable technique to construct a nanofiber matrix composite electrolyte with boosting Na+ transport and interfacial compatibility for SIBs. Benefitting from the salt dissociation and selective cation conduction synergistic effect of the acylamino, carbonyl, and ester groups in the low‐cost copolymer synthesized from 2‐(methacryloyloxy)ethyl acetoacetate and N,N′‐methylenebisacrylamide, a facilitating of Na+ transport at extreme temperatures is realized. Besides, flexible flame retardance ceramic SiO2 nanofibers greatly enhance high‐temperature safety. The ultrathin functional AlF3 layer generated by binder‐free magnetron sputtering suppresses the dendrites, eliminating the interfacial issues between the electrolyte and anode, which is proved by 5500 h of ultrasteady plating/stripping. Superior ionic conductivity of 0.153 mS cm−1 at −30 °C implies fast Na+ transport, which is further evidenced by molecular dynamics simulations. Rate performance at 0.05–10 C from −30 to 130 °C further demonstrates the excellent electrochemical performance of the electrolyte. This work provides encouraging guidance for high‐safety SSEs with rapid Na+ conduction for SIBs operating at extra‐wide temperatures. A rigid‐soft coupled nanofiber matrix composite electrolyte with salt dissociation and selective cation transport boosting is constructed by an in situ UV curing method. The flexible electrolyte exhibits low‐temperature adaptability and high‐temperature safety properties. What is more, the ultrathin binder‐free AlF3 layer on the electrolyte surface constructed by magnetron sputtering ensures a dendrite‐free anode.