Carbon nitride (CN) photocatalysts have attracted much attention due to their excellent photocatalytic properties. And hydrothermal fluorination is a common method to improve the photocatalytic effect of CN photocatalyst. Here, the influence of the band gap was first revealed of fluorination and hydroxylation of CN photocatalyst based on the first theoretical principle. Here, the effect of fluorination and hydroxylation on the CN band gap was discussed for the first time using the first theoretical principle. With F atoms and OH doping, the band gap of CN was significantly improved, conduction band and valence band moved up. Then, F--CN photocatalyst with F atoms and OH was successfully synthesized by a hydrothermal fluorinated method. Next, the reasons why F--CN photocatalyst was more effective than that of traditional CN photocatalyst were fully discussed. From the photocatalytic effect of photocatalyst (12,593.2 μmol g 1 h 1 to the morphology (super-small nanosheets), structure (homojunctions), composition (metal-free), specific surface area (54.1 m2/g), visible light absorption response (AQE is 10.9% at 420 nm) and photo-induced carrier life (14.13 ns). Therefore, this work has a great guiding effect on the development of CN photocatalyst.