The structure, electronic properties and optical properties of zinc blende CdS and CdS∶M (M=Mg, Ni) were studied systemically from density functional theory (DFT) based on first-principles pseudopotential calculations. It showed that the M-doped material had a smaller lattice constant, which resulted in a local lattice distortion. The band structure and density of states (DOS) were further calculated, which indicated that CdS∶Ni was half metallic ferromagnetic semiconductor while CdS∶Mg was semiconductor with bigger band gap due to the introduction of Ni 3d and Mg 3s. The optical property was also studied from the calculated absorption coefficients, which showed that the absorption peaks changed obviously at the visible light wavelength area for the M-doped CdS. Furthermore, the absorption ranges was extended to even higher wavelength region for the Ni-doped CdS.