CdS thin films are deposited on the substrate temperature of 300 ℃ by magnetron sputtering. The deposited films are annealed at temperature of 370℃, 380 ℃ and 390 ℃ respectively in two atmosphere of dry air and CdCl2 source +dry air. The morphology, structure and optical properties before and after thermal annealing are investigated. The results show that annealed in dry air, grain size, surface roughness and visible light transmittance change is not obvious, the optical band gap increases with temperature increasing. Annealed in CdCl2 source + dry air, recrystallization and grain growth are significant, surface roughness increases, visible light transmittance and optical band gap decreases with temperature increasing. Analysis revealed that the performance is the results of recrystallization temperature and the band tail states doping concentration changes at different annealing conditions.CdS thin films were deposited on the substrate at temperature of 300 ℃ by magnetron sputtering. The deposited films were annealed at temperature of 370℃, 380 ℃ and 390 ℃, respectively, in either atmosphere of dry air or a mixture of CdCl2 and dry air. The morphology, structure and optical properties before and after thermal annealing were investigated. The results show that for samples annealed in dry air, the variations of grain size, surface roughness and visible light transmittance with annealing temperature are not obvious, while the optical band gap increases with temperature increasing. Annealed in the mixture of CdCl2 and dry air, recrystallization and grain growth in the samples are significant. Surface roughness increases with temperature increasing, on the contrary, visible light transmittance and optical band gap decreases. These phenomena are the results of changes in the temperature of recrystallization and doping concentration in the band tail states at different annealing atmosphere.