<p>Fe-based Prussian blue analogs are promising positive electrodes for sodium-ion batteries due to their open framework and high theoretical capacity, yet suffer from structural degradation caused by interstitial water, lattice distortion, and irreversible phase transitions. Here we show a selection principle for alkali metal ion lattice engineering, based on geometric matching, hydration energy, and Fe-N bonding strength, to design high-energy, long-life positive electrodes. We demonstrate that immobilizing Cs⁺ ions at the alkali metal sites enhances the structural and electrochemical stability of iron-based Prussian blue analogs. The introduced Cs⁺ pillars alleviate lattice contraction, suppress framework distortion, enhance Fe-N covalency, and reduce crystal water content. These modifications promote sodium ion diffusion and charge transfer kinetics, especially across phase transition regions. Consequently, the optimized positive electrode delivers a high capacity retention of 81.48% after 1000 cycles at 750 mA g⁻¹ and maintains 77.55% of its capacity at a high specific current of 7500 mA g⁻¹. It also exhibits good air stability and wide-temperature performance. A proof-of-concept pouch cell demonstrates its practical viability. This work establishes a viable selection principle for alkali-metal-site regulation, advancing the development of long-life, high-energy Prussian blue positive electrodes for large-scale sodium-ion battery applications.</p>