Plane-wave-based pseudopotential density functional theory (DFT) calculations are used to characterize the doping effect of S substituting for O in anatase TiO2. Through band structure calculation, a direct band gap is predicted in TiO2-xSx. Electronic structure analysis shows that the doping S could substantially lower the band gap of TiO2 by the presence of an impurity state of S 3p on the upper edge of the valence band. Excitations from the impurity state of S 3p to the conduction band may be responsible for the red shift of the absorption edge observed in the S-doped TiO2. The band gap lowering and red shift of the absorption edge are found to increase as the sulfur concentration increases.