<p>Prussian blue nanoparticles (PBNPs) have emerged as versatile nanozymes with reactive oxygen species (ROS)-scavenging capabilities, predominantly applied in antioxidant therapies. In this work, we present a combined theoretical and experimental study demonstrating that modulating Fe coordination environments can fundamentally reconfigure PBNPs’ catalytic properties, enabling ROS generation and pro-oxidative functionality. Ab initio molecular dynamics revealed different H₂O₂ lysis mechanisms at Fe sites with varying coordination numbers: Low-coordinated center (FeN₄) induced hydrogen atom transfer to form Fe=O species, while high-coordinated FeN₅ generated ·OH radicals via H⁺-assisted homolysis under acidic conditions. Guided by calculations, Cs-doped PBNPs (Cs-PBs) with elevated coordination numbers were synthesized via alkali cation stoichiometric control, leveraging high distribution coefficient and low hydration energy of Cs⁺. Experimental results confirmed radical generation in Cs-PBs aligned with theoretical predictions. The size-optimized Cs-PBs demonstrated ultrahigh peroxidase-like activity (1182.26 U·mg^-1) and outperformed ROS generating properties in both pollutant degradation and chemodynamic therapy. This work redefines PBNPs’ catalytic potential beyond conventional antioxidant roles, and lays the foundation for innovative environmental and therapeutic solutions.</p>