Defects modulation usually has great influence on the electronic structures and activities of photocatalysts. Here, porous structured graphitic carbon nitride materials with large amount of defects are obtained through facile 5min thermal treatment in air without additional reactants. The resultant materials show remarkably extended light absorption in the visible light region. Theoretical calculations indicate that the distinctive origin of red-shifted intrinsic light absorption edge and newly occurred light absorption edge are attributed to cyano groups and nitrogen vacancies, respectively. Compared to pristine graphitic carbon nitride, the optimally modified material shows greatly enhanced photocatalytic hydrogen evolution rate by 21.5 times under λ > 440nm and the responsive wavelength is extended from 450nm to 650nm. This work is expected to provide guidance for rational design of graphitic carbon nitride and inspire similar attempts for the modification of nanomaterials. Display omitted •A 5 min thermal treatment method is developed to modify graphitic carbon nitride.•Greatly extended visible light absorption is realized through defects introduction.•Distinctive defects have different influence on visible light modifications.•The modified materials show highly increased visible light activities.