The energy band structures of Fe-doped, S-doped and Fe-S co-doped ZnO are investigated using first-principles pseudo-potential method based on density functional theory. The density of states and optical properties are compared and analyzed as well. The results indicate that the lattices of ZnO distort after doping. The energy gap of ZnO decreases because of S doping. The several impurity energy levels are introduced after Fe doping and Fe-S co-doping, which lead to the significant increase of photon energy absorption in visible and ultraviolet regions, the expansion of spectral response range and the improvement of ZnO photocatalytic performance. In the Fe-S co-doped ZnO system, the interaction between Fe ions and S ions leads to the decrease of impurity energy levels in the band gap, and the absorption coefficient is between Fe doped and S doped ZnO.