Based on the plane wave ultra soft pseudopotential method of density functional theory, the geometric structure, energy band structure, electronic density of states and optical properties of wurtzite CdS containing Cd vacancy defect and wurtzite CdS containing S vacancy defect were studied by the firstprinciple. According to calculation and analysis, the CdS systems containing Cd vacancy defects are all ptype semiconductors and the transition modes of CdS systems containing S vacancy defects are all changed from direct transition to indirect transition. The density of states and total energy of CdS system with Cd and S vacancy defects decrease. The static dielectric constant of vacancy CdS system is higher than that of intrinsic CdS system, and increases with the increase of vacancy concentration. The vacancy defect system of Cd is more obvious, and the polarization ability is significantly improved. The CdS system of Cd vacancy has obvious absorption in infrared band compared with the intrinsic CdS system, and the CdS system of S vacancy has obvious absorption in visible light compared with the intrinsic CdS system.