In this paper, the electronic structure and optical properties of zinc blend-type compounds CdS and ZnO_1-xS_x (x=0, 0.25, 0.5, 0.75, 1) are numerically analyzed by means of the plane wave ultrasoft pseudo-potential method with the generalized gradient approximation (GGA) under the density functional theory (DFT). The lattice parameters, the band structure, the density of states and the optical conductivity are obtained. The optical properties including the dielectric function, the conductivity, the absorption spectrum and the energy-loss spectrum are discussed. It shows that the zinc blend-type compounds CdS and ZnO_1-xS_x are both direct band gap materials; the crystal constants increase with the quantity of S doping ZnO; the ZnO_1-xS_x has a wider band gap than CdS and can be expected to provide a high quality buffer layer for high efficiency Cu(In_1-xGa_x)Se₂ (CIGSe) solar cells by improving the surface quality of absorption layer and decreasing the absorption loss.