Fluoride solid electrolytes stand out as promising candidates for high-energy-density solid-state batteries owing to their inherent structural and air stability. However, their application in sodium based batteries has been hindered by their low ionic conductivity. Here, we propose a heterostructured fluoride-based solid electrolyte engineered by grain boundary softening and bonding for sustainable Na metal batteries. By leveraging a low-melting-point chloride eutectic mixture NaCl-1.1AlCl3 (NA), we achieve the in situ activation of sodium ion transport at the grain boundaries of Na3GaF6 (NGF), with the formation of a molten salt interphase primarily composed of NaAlCl4. This process effectively softens the edges of NGF and heals the intergranular voids. The NA decorated NGF electrolyte exhibits the significantly enhanced ionic conductivities of 2.7 × 10-4 S/cm at 60°C and 2.45 × 10-5 S/cm at room temperature, representing two orders of magnitude improvement over pure NGF. The chloride shell does not degrade the wide electrochemical window and air stability of NGF. This electrolyte relies on a unique "self-protection" mechanism when cycling with sodium metal anode. The molten salt phase can evolve into a compact NaCl protective shell and an Na-Al conductive inner layer with tight contact with Na anode. This dual-layer structure mitigates the side reaction with NGF host, homogenizes the current flow, and suppresses the growth of sodium dendrites. The corresponding Na//Na symmetric cells display the small voltage polarization and stable cycling performance for over 1000 hours. The solid-state Na//FeF3 batteries deliver a high capacity of 308 mAh/g and sustain for at least 100 cycles with high capacity retention based on conversion reaction.