The energy band structure, density of electronic state and optical properties of Ce-doped anatase titanium dioxide（TiO2）, S-doped anatase TiO2 and Ce/S co-doped anatase TiO2 are investigated with first-principles based on density functional theory（DFT）. The results show that the lattice constant is enlarged due to doping and the forbidden band is also decreased. For the Ce/S co-doped anatase TiO2, its forbidden bandgap is the smallest of all because of the synergy of S-3p and Ce-4f electronic orbits, which leads to the red shifts of the absorption spectra. Furthermore, Ce atom has two kinds of variable valence (Ce4+ and Ce3+), which can prevent the recombination of electrons and holes. The photocatalytic properties of TiO2 can be improved by Ce/S co-doping.