Highly efficient platinum-group metal (PGM)-free electrocatalysts are essential for the large-scale utilization of Zn-air batteries (ZABs). Herein, we report the simple fabrication of a single atomic PGM-free electrocatalyst, Fe-SA/N-C, via a post-synthetic electrostatic absorption (PSEA) strategy. The single Fe atoms are anchored on the three-dimensiaonal (3D) porous carbon with adjacent N atoms, forming atomic Fe-N₄ active sites. Fe-SA/N-C exhibits excellent ORR activity in 0.1 mol L⁻¹ KOH aqueous solution (E_1/2 = 0.92 V) and 0.5 mol L⁻¹ H₂SO₄ aqueous solution (E_1/2 = 0.77 V), superior to those of commercial Pt/C (0.85 and 0.79 V, respectively). As a proof of concept, homemade liquid ZAB with Fe-SA/N-C catalyst displays outstanding discharging specific capacity and peak power density, outperforming the commercial Pt/C. According to the density functional theory calculation, the Fe-N₄ sites with graphitic N dopant can improve the activation of intermediates and decrease the energy barrier of the rate-determining step. This work highlights new insights for the experimental and theoretical guidance of PGM-free electrocatalysts and prescribes a general strategy for the rational design of PGM-free electrocatalysts used in ZABs.