Thermal oxidation of bulky carbon nitride (C3N4, CN) to expose more active sites is an important method to improve the activity of the as-prepared CN nanosheets. Unfortunately, the yield of thermal oxidation is low. Herein, we report dual-deficient CN (DDCN) ultrathin nanosheets engineered with nitrogen vacancies and cyano groups by two-step bottom-up thermal polymerization of belt-like melamine with a high yield of 65%. Featured with abundant exposed active sites, short charge migration distance, wide visible-light absorption, quick charge separation/transfer, enhanced oxygen adsorption ability and 2e oxygen reduction reaction selectivity, the DDCN exhibits a high H2O2 production rate of ∼1031 μmol g1 h1, which is 19.5 times that of CN (53 μmol g1 h1) under visible light irradiation (λ ≥ 420nm). Specifically, the apparent quantum yield (AQY) of DDCN reached 10.7% at 420nm. This study provides a facile dual-defect engineering method to develop highly efficient ultrathin CN-based photocatalysts.