Carbon nitride (C₃N₄) holds great promise for photocatalytic H₂O₂ production from oxygen reduction. In spite of great research efforts, they still suffer from low catalytic efficiency primarily limited by the fast recombination of photogenerated charge carriers. In this work, we report the multiscale structural engineering of C₃N₄ to significantly improve its optoelectronic properties and consequently photocatalytic performance. The product consists of porous spheres with high surface areas, abundant nitrogen defects, and alkali metal doping. Under visible light irradiation, our catalyst shows a remarkable H₂O₂ production rate of 3,080 μmol·g⁻¹·h⁻¹, which is more than 10 times higher than that of bulk C₃N₄ and exceeds those of most other C₃N₄-based photocatalysts. Moreover, the catalyst exhibits great stability, and can continuously work for 15 h without obvious activity decay under visible light irradiation, eventually giving rise to a high H₂O₂ concentration of ca. 45 mM.