Graphite phase carbon nitride (g-C3N4) quantum dots have received much attention due to its good stability, water solubility, biological compatibility, non-toxicity as well as strong fluorescence characteristics. In order to enhance the light absorption and improve photocatalytic activities of the g-C3N4 quantum dots, theoretical studies are carried out on the O and S atoms doped (g-C3N4)6 quantum dots. First-principles calculations based on the density functional theory and time dependent density functional theory are performed to investigate the geometries, electronic structures and ultraviolet visible absorption spectra of O and S atoms doped (g-C3N4)6 quantum dots. The results show that the highest electron occupied molecular orbital-the lowest electron unoccupied molecular orbital (HOMO-LUMO) energy gap of doped (g-C3N4)6 quantum dots is significantly reduced though the CN bond lengths closely related to the impurities only change slightly. The calculated formation energies indicate that the O-doped (g-C3N4)6 quantum dots are more stable, and the O atom tends to substitute for N atom at the N3-site, while the S atoms prefer to substitute for N atom at the N8-site. The simulated spectra indicate that the doping of O and S in (g-C3N4)6 could improve the light absorption. Not only the absorption peaks are extended from the UV to the infrared region (e.g. 200-1600 nm), but also the corresponding absorption intensities are enhanced significantly by doping the O or S atoms with the appropriate concentration. The increase of proper impurity concentration will lead to a pronounced red shift in light absorption. The effect of doping site on the optical absorption property of (g-C3N4)6 quantum dots shows that the absorption intensity is mainly affected in the visible range, however, besides the influence on the absorption intensity, the light absorptions of some structures are also affected beyond 800 nm. Overall, the O atoms and S atoms have a substantially similar effect on the light absorption of the (g-C3N4)6 quantum dots, while the effects of these impurity atoms are different in the long wavelength region. Oxygen doping is better than sulfur doping in the absorption of (g-C3N4)6 quantum dots by comparing the doping of O and S. These first-principles studies give us a method to effectively improve the light absorption of g-C3N4 quantum dots, and could provide a theoretical reference for tuning its electronic optical properties and applications.