Photocatalytic oxygen reduction has garnered attention as an emerging alternative to traditional anthraquinone oxidation process to synthesize H2O2. However, despite great efforts to optimize photocatalyst activity, the formation rate has been largely limited by the deficient accessibility of the photocatalysts to sufficient O2 in water. Here we boost the reaction by reporting an air-liquid-solid triphase photocatalytic system for efficient H2O2 generation. The triphase system allows reactant O2 to reach the reaction interface directly from the ambient atmosphere, greatly increasing the interface O2 concentration, which in turn simultaneously enhanced the kinetics of formation constant and suppressed the unwanted electron-hole recombination and the kinetics of H2O2 decomposition reaction. Compared with a conventional liquid-solid diphase reaction system, the triphase system enables an increase in H2O2 formation by a factor of 44. The triphase system is generally applicable to fundamentally understand and maximize the kinetics of semiconductor-based photocatalytic oxygen reduction for H2O2 generation.