The geometry structure, formation energy and electronic property of NS codoped anatase TiO2 (NSTiO2) were investigated by the density functional theory (DFT) of first-principles. The result indicated that the O-poor growth condition is beneficial to the formation of NSTiO2 except the NTiSTiTiO2 with non-sensitivity to the atmosphere. The visible-light photocatalytic activity of NSTiO2 was influenced by many factors including the value of energy gap (Eg), the distribution of impurity level, the property of impurity level, the location of Fermi level and the energy in the edges of band gap. The impurity level localized in the top of valence band reduced the energy gap, leading to the absorption of visible light. Besides, the impurity level delocalized in the band gap reduced the critical energy of available photon, which further strengthened the absorption of visible light. Moreover, the bent impurity level was the efficient separation center of photogenerated electron and hole, while the flat impurity level was the combination center of photogenerated electron and hole. Further, the valence band and conduction band of NSTiO2 moved toward the low energy region (down shift), resulting in the stronger oxidizability of photogenerated hole in the top of valence band and weaker reducibility of photogenerated electron in the bottom of conduction band. This indicated that the dominant oxidant species during photocatalytic reaction for NSTiO2 was hole and OH radical instead of O2− ion.