Atomically dispersed metals stabilized by nitrogen elements in carbon skeleton hold great promise as alternatives for Pt-based catalysts towards oxygen reduction reaction in proton exchange membrane fuel cells. However, their widespread commercial applications are limited by complicated synthetic procedures for mass production. Herein, we are proposing a simple, green mechanochemical approach to synthesize zeolitic imidazolate frameworks precursors for the production of atomically dispersed “Fe-N₄” sites in holey carbon nanosheets on a large scale. The thin porous carbon nanosheets (PCNs) with atomically dispersed “Fe-N₄” moieties can be prepared in hectogram scale by directly pyrolysis of salt-sealed Fe-based zeolitic imidazolate framework-8 (Fe-ZIF-8@NaCl) precursors. The PCNs possess large specific surface area, abundant lamellar edges and rich “Fe-N₄” active sites, and show superior catalytic activity towards oxygen reduction reaction in an acid electrolyte. This work provides a promising approach to cost-effective production of atomically dispersed transition metal catalysts on large scale for practical applications.