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An in-depth understanding of the microscopic reaction mechanism on a nonmetal-doped catalytic system at the atomic level is one of the critical approaches to developing new efficient catalysts. Herein, the effects of S-doping on melon-based carbon nitride (CN) for the photocatalytic selective oxygen reduction reaction (ORR) have been comprehensively investigated by first-principles calculations. The configurations, electronic properties, optical properties, and the reaction performance of the S-doped melon-based CN have been studied and discussed. The results demonstrate that the decoration with S atoms exhibited substantial effects, involving the redistribution of the charge density and tuning of the bandgap, which promote the photocatalytic selective ORR activity. Accordingly, O2 is activated on the S-doped system with about 0.4 e of charge obtained from catalytic surfaces, leading to the thermodynamically feasible H2O2 and H2O formation, which is in good agreement with the experimental results. Our results provide theoretical insights into the design and development of polymeric carbon nitride (PCN) as well as other metal-free photocatalysts for the selective ORR.