The efficacy of single-site defect engineering in graphitic carbon nitride ($\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4$) is often compromised by the inevitable creation of charge recombination centers. To overcome this limitation, we propose a synergistic dual-site defect strategy and develop a high-pressure and high-temperature (HPHT) method to concurrently introduce oxygen dopants and nitrogen vacancies into $\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4$. During HPHT treatment, adsorbed oxygen atoms are embedded into the lattice, while adjacent nitrogen atoms are expelled to form vacancies. Such dual-defects induce slight band-gap narrowing of $\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4$ while significantly promote charge transfer and separation, thereby boosting photocatalytic hydrogen evolution. Theoretical calculations confirm that the oxygen dopants (p-type doping) and nitrogen vacancies (n-type doping) act as donor-acceptor pairs. This charge compensation effect prevents the formation of localized impurity states in the gap but instead modifies the band edges to enhance the oxidation capability and facilitate the spatial separation of electron and hole carriers. Benefiting from these advantages, the optimized $\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4$ exhibits a twofold enhancement in photocatalytic activity, accompanied by a marked increase in carrier concentration. This work provides a novel strategy and fundamental insights for synthesizing highly efficient defect-engineered photocatalysts.

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• Charge dynamics
• Hydrogen production