Prussian blue analogues (PBAs) are recognized as promising cathode materials for sodium-ion batteries (SIBs) due to their facile synthesis, low-cost, high capacity, and environmental friendliness. However, high water content (>10 wt%) in the framework and unsatisfactory structural stability of PBAs are still the bottlenecks for industrial applications. Herein, interstitial K-doping is employed to minimize the interstitial water and enhance the structural stability of NaFeMn[Fe(CN)] (FeMnPBA), thereby boosting the sodium storage performance. The 3% K-doping (K-FeMnPBA3) demonstrates a much-reduced water content of 6.9%, accompanied by a notably enhanced capacity of 139.1 mAh g at 100 mA g and a remarkable capacity retention of 77.1% after 700 cycles. Furthermore, the K-FeMnPBA3/hard carbon (HC) pouch cell achieves a stable cyclability with 82.6% capacity retention after 600 cycles. This research offers valuable insights into low-water PBAs for practical applications in SIBs.