Fe-N ₅ single-atom catalysts (SACs) hold great promise for water decontamination, however, the fundamental relationship between their high coordination shell environment and catalytic performance in Fenton-like reactions remains poorly understood. Here, we precisely regulate the high coordination shell defects of a model SAC with well-defined axial Fe-N ₅ configurations to elucidate the impact of remote interactions on peroxymonosulfate (PMS) activation. Experimental and theoretical studies confirm that remote modulation of Fe-N ₅ sites through high coordination shell defects profoundly enhance Fenton-like catalytic activity, enabling FeN ₅ -SD ₂ to achieve a turnover frequency (TOF) value of 0.338 min⁻ ¹ , surpassing state-of-the-art SACs. Our findings reveal a critical volcano-type correlation between defect content and catalytic efficiency, where coordinated modulation of Fe d -band center positioning and PMS adsorption energetics governs reaction dynamics. Only the FeN ₅ -SD ₂ configuration with an optimal level of defects density and moderate adsorption energy enables sufficient O-O bond elongation in PMS to lower the energy barrier for selective singlet oxygen ( ¹ O ₂ ) evolution. This study unveils the mechanistic role of higher coordination shell defects in regulating Fe-N ₅ active sites and introduces a well-defined model to investigate the structure–property correlations of higher coordination shells in SACs for Fenton-like reactions.