Abstract: TiO2 is an excellent photocatalyst for its chemical stability，nontoxicity and high photocatalytic reactivity.However，TiO2 is activated only by ultraviolet light because it is a large band-gap semiconductor. Doping with nitrogen can definitely shift the absorption spectra of TiO2 toward visible light regions，but the mechanism is still under disputed.In this paper，these mechanism of N-doped TiO2 photocatalyst with visible light activity is introduced，included bandgap narrowing theory，impurity energy level theory and oxygen deficiency theory，and point out the trend of the mechanism research of N·doped TiO2.

Key words: visible light, photocatalysis, bandgap narrowing, impurity energy level, oxygen deficiency, 杂质能级, 氧空缺