Herein, carbon-defect engineering and 2-dimensional engineering are integrated into g-C3N4 at once, allowing one-pot output of ultrathin g-C3N4 photocatalyst by a thermal-triggering in-situ gas-shocking process using endogenous gas (CO2, H2O, and NH3) from urea solution. The optimal photocatalyst U1W1-CNS presents an ultrathin structure (2 nm thickness) with abundant carbon-defects in a porous state, thereby endowed with outstanding structural property (191.4 m(2) g(-1), 0.61 cm(3) g(-1)). Meanwhile, benefited from the emergent carbon defects, the conduction band of U1W1-CNS can be shifted to a higher energy level, thus contributing to stronger reduction ability which was comprehensively confirmed by experimental evidence and DFT calculation, and the hydrophilicity of U1W1-CNS is further improved by more exposed edge amino moieties. As expected, U1W1-CNS affords 57 folders of hydrogen production (10.14 mmol h(-1) g(-1)) efficiency, and greater degradation efficiency for different organic pollutants RhodamineB (k = 0.0311 min(-1)), tetracycline (k = 0.0135 min(-1)), norfloxacin (k = 0.0091 min(-1)), ciprofloxacin (k = 0.012 min(-1)), and levofloxacin (k = 0.0078 min(-1)).