The geometry structure, electronic structure and optical properties of zinc bende CdS:M (M = Ag,Zn) were studied systemically using density functional theory based on first-principles ultrasoft pseudopotential method and GGA.The relationship between the distribution of the electronic states and their structures has been analyzed. The dielectric function and the complex refractive index function on doping CdS system have been obtained. The calculation results showed that the hole concentration of Ag-doped CdS is increased, enhanced the conductivity of the material. But the carrier concentration of Al-doped CdS was not changed; the optical band gap were narrowed for Ag-doping, Zn-doping CdS. The mechanism of luminescence has been explained in terms of an analysis of the dielectric function and the complex refractive index function.