The geometrical structures of Ag-doped ZnS, that with Zn vacancies and pure zinc blend were optimized by means of plane wave pseudo-potential method (PWP) with generalized gradient approximation (GGA). The electronic structure and optical properties of the three systems were calculated and the reason why p type ZnS is difficult to form was given theoretically. Equilibrium lattice constant, band structure, electronic structures and optical properties were discussed in detail. The results reveal that,in Ag-doped ZnS and Zn vacancy systems, due to the introduction of the defect level, the band gap is reduced and electronic transition in the visible region is obviously enhanced.