The band structure, density of states and optical properties of pure ZnO, S single-doped， La single-doped and S-La co-doped ZnO were studied by first-principles method based on the generalized gradient approximation. After ZnO doped with S, the valence band and conduction band transfer to low energy at the same time, which leads to the reduction of band gap. The band gap of ZnO doped with La reduces because of the impurity energy level at the base of the conduction band. The localization of S-La co-doped ZnO results in the weakening of La localization, which indicates that the donor energy level of La becomes shallower due to the influence of S 3p state, thus reducing the band gap. After doping, the band gap of ZnO decreases obviously, which increases the absorption ability of ZnO in visible light region, and further produces more photo-electric charge carriers to improve the photocatalytic activity of ZnO. Compared with pure ZnO, the doped ZnO has lower absorption coefficient and reflection coefficient, and stronger transmission ability, which provides a potential theoretical basis for the application of ZnO as transparent conductive oxide in solar cells.