The buffer layer of CdS thin films for solar cells were prepared in the general load slides by a simple chemical bath deposition (CBD). By changing the experimental conditions such as the pH value of the solution and the annealing temperature, the optimum preparation technology of CdS thin film was discussed. The thin film samples were characterized by X - ray diffraction, UV - Vis spectrophotometer and electrochemical workstation. The experimental results showed that the best experimental conditions for the preparation of CdS thin films with good uniformity, density and coverage were as the reaction temperature being 70 ℃, the solution pH value being 10 and the subsequent heat treatment at 350 ℃ being 1 hour. Under this condition, the visible light transmittance of the film was higher with obvious phenomenon of the visible light transmittance. By calculating, the band gap of the films was 2.35 eV which was very close to the theoretical value(2.42 eV).