Modifying the electronic structure of photocatalysts is an important approach for increasing the separation efficiency of photogenerated carriers and activating molecular oxygen into superoxide radical (O-.(2)-) for contaminants removal. Herein, based on the density functional theory (DFT) calculations, we found that doping F- can not only narrow the bandgap of graphitic-phase carbon nitride (g-C3N4), but also decrease the electronic localization, extend the pi conjugated system and increase the in-planar electron density, resulting in promoting the molecular oxygen activation of g-C3N4. The experiments indicated that the fluorinated ultrathin g-C3N4 was favorable for activating molecular oxygen into O-.(2)- under visible light irradiation, which could be ascribed to the relatively higher conduction band position and more negative surface adsorption energy for O-2 as well as advantageous charge accumulation on adsorbed O-2 as compared to ultrathin g-C3N4. By virtue of the synergic effect of enhanced adsorption ability to O-2, effective separation of photogenerated carriers and more reductive photoexcited electrons derived from the ultrathin structure and F- associates, the fluorinated ultrathin g-C3N4 sheets revealed promoted solar driven photocatalytic activity for formaldehyde (HCHO) oxidation. This study could provide new insights into the mechanistic comprehension of molecular oxygen activation reaction and design of low cost noble metal-free modified ultrathin layered photocatalysts with for environmental remediation.