Chlor-alkali electrolysis with oxygen depolarized cathode (ODC), which can save up to 30 % in energy, primarily involves two electrochemical reactions: the chlorine evolution reaction (CER), and the oxygen reduction reaction (ORR). However, electrocatalysts used in this field is the platinum-group metals (PGMs), which significantly increase the preparation cost. Herein, we synthesize Fe single-atom anchored on carbon nanotubes (Fe-N@CNTs) via a pre-coordination strategy as an active and durable ORR catalyst. The resulting Fe-N@CNTs displays excellent ORR performance with a large half-wave potential (E1/2) of 0.905 V and excellent durability over 30,000 CV test in 0.1 M KOH electrolyte. Projected density of states (PDOS) analysis demonstrates that geometric strain lowers the d-band center of Fe, weakening the adsorption strength of ORR intermediates at Fesingle bondN4 active sites. Density functional theory (DFT) calculations show that geometric strain effects reduce the potential determining step (PDS) energy barrier and optimize the binding strength of *OH of ORR. Both Fe-N@CNTs-based aqueous Zn-air battery (A-ZAB) and quasi-solid-state Zn-air battery (QSS-ZAB) display excellent charge/discharge cycling stability with small charge/discharge voltage gaps over 1700 h and 100 h, respectively. Importantly, Fe-N@CNTs||RuO2 chlor-alkali flow cell requires low cell potential of 1.58 V to reach 300 mA cm−2 at 80 °C, and delivers excellent stability over 230 and 110 h at 100 and 300 mA cm−2, respectively. Fe-N@CNTs||RuO2 maintains approximately 100 % caustic current efficiency at the current density of 300 mA cm−2. This work contributes to the development of stable, uniformly dispersed atomic-scale metal ORR catalysts and expands their applications in chlor-alkali electrolyzer.