As graphitic carbon nitride (gCN) can only absorb visible light with wavelength < 450 nm, and the recombination of photo-generated carriers is quick, resulting in its moderate photoreactivity. Herein, flower-like gCN assembly from porous nanosheets with nitrogen vacancies was prepared by calcination of melamine-cyanuric acid (MCA) supramolecular aggregates at 500 degrees C. NO oxidation was used to evaluate the photoreactivity of the prepared gCN in a continuous-flow reactor using a visible LED lamp (lambda >= 400 nm) as the light source. We systematically studied the effect of calcination time (2-14 h) on the structure, property and photocatalytic performance of the prepared flower-like gCN. Enhanced visible photoreactivity of flower-like gCN was observed with extension the calcination time, and the sample calcined for 10 h (F10 sample) exhibits the highest NO removal rate (59.7%), which is much higher than that of bulk gCN (45.8%, B10 sample) which was prepared by calcination of melamine at the same temperature for 10 h. The improved photocatalytic activity of flower-like gCN is ascribed to the condensed pi-pi layer stacking, breaking of intraplanar hydrogen bonds, enlarged BET surface area, and formation of nitrogen vacancies, which result in a broadened visible responsive range and improved separation of the photo-generated carriers.