With a series of merits, Prussian blue analogs (PBAs) have been considered as superior cathode materials for sodium‐ion batteries (SIBs). Their commercialization, however, still suffers from inferior stability, considerable Fe(CN)6 defects and interstitial water in the framework, which are related to the rapid crystal growth. Herein, a “water‐in‐salt” nanoreactor is proposed to synthesize highly crystallized PBAs with decreased defects and water, which show both superior specific capacity and rate capability in SIBs. The air‐stability, all‐climate, and full‐cell properties of our PBA have also been evaluated, and it exhibits enhanced electrochemical performance and higher volume yield than its counterpart synthesized via the water‐based co‐precipitation method. Furthermore, their highly reversible sodium‐ion storage behavior has been measured and identified via multiple in situ techniques. This work could pave the way for the PBA‐based SIBs in grid‐scale energy‐storage systems. A facile, cost‐efficient, and general programmable ball‐milling synthesis process without any additives was designed to prepare highly crystalline MnHCF‐S‐170. The constructed sodium‐ion batteries (SIBs) based on the above MnHCF‐S‐170 can adapt to the complex and fickle working environment, and demonstrate possible practical applications in large‐scale energy‐storage systems.