H2O2 yield is substantially hindered by the four-electron competitive reaction and limited oxygen-capture ability of PCN. We herein demonstrated a Rb+-modified PCN photocatalyst (CNR-0.5) for high-efficient H2O2 production, in which the introduction of Rb+ ions creates abundant edge active sites within –C[triple bond, length as m-dash]N/–OH groups of the triazine ring. The hydrophilicity and O2 adsorption capacity of the PCN catalyst were significantly improved. As a result, CNR-0.5 achieved a relatively high H2O2 yield (2225.05 µmol g−1 h−1), which is 37.58 times higher than that of pristine PCN under white light-emitting diode (LED) light. Experimental and theoretical results demonstrated that CNR-0.5 followed a two-step single-electron O2 reduction reaction pathway in the H2O2 photogeneration reaction. Additionally, the AQE of CNR-0.5 was 24.52% at 385 nm in an ethanol system with O2 injection. The excellent photocatalytic H2O2 production performance of CNR-0.5 in a real water body broadens the path for practical applications of PCN materials in photocatalytic H2O2 production.