Efficient generation of active oxygen-related radicals plays an essential role in boosting advanced oxidation process. To promote photocatalytic oxidation for gaseous pollutant over g-C₃N₄, a solid–gas interfacial Fenton reaction is coupled into alkalinized g-C₃N₄-based photocatalyst to effectively convert photocatalytic generation of H₂O₂ into oxygen-related radicals. This system includes light energy as power, alkalinized g-C₃N₄-based photocatalyst as an in situ and robust H₂O₂ generator, and surface-decorated Fe³⁺ as a trigger of H₂O₂ conversion, which attains highly efficient and universal activity for photodegradation of volatile organic compounds (VOCs). Taking the photooxidation of isopropanol as model reaction, this system achieves a photoactivity of 2–3 orders of magnitude higher than that of pristine g-C₃N₄, which corresponds to a high apparent quantum yield of 49% at around 420 nm. In-situ electron spin resonance (ESR) spectroscopy and sacrificial-reagent incorporated photocatalytic characterizations indicate that the notable photoactivity promotion could be ascribed to the collaboration between photocarriers (electrons and holes) and Fenton process to produce abundant and reactive oxygen-related radicals. The strategy of coupling solid–gas interfacial Fenton process into semiconductor-based photocatalysis provides a facile and promising solution to the remediation of air pollution via solar energy.