The cadmium sulfide (CdS) semiconductor thin film for solar cells were prepared by using a chemical bath deposition method. The factors which had influence on film formation, film structure and its optical properties were tested and studied preliminary. The result showed that the pH value of the reaction solution and the annealing temperature were the main factors that had influence on film formation. In the experiment, the pH value was controlled in the range from 10.5 to 10.8 and the optimum annealing temperature was 400 ℃. In addition, by dropping some CdCl₂ solution and dripping it onto the surface of the film in annealing, the transmittance of the film in the region of visible light can be further improved.