CdS thin films for the buffer layer of Cu(In,Ga)Se2 solar cells were prepared by chemical bath deposition at different ammonia dosages. The initial concentration, pH value and ion product of the reaction particles in the mixed solution were calculated according to the chemical equilibrium kinetics. The film thickness, surface morphology, crystal structure, external quantum efficiency and photoelectric conversion efficiency of the samples were analyzed by step tester, SEM, XRD, EQE and IV tester. The results show that with the increase of ammonia amount, the homogeneous reaction rate reduces because the free Cd2+ concentration decreases, and the heterogeneous reaction rate increases because the Cd(OH)2(NH3)n density adsorbed on the substrate surface increases; the crystal structure of CdS thin films changes from cubic phase to hexagonal phase because the cubic phase is formed by the homogeneous reaction, and the hexagonal CdS thin film is formed by the heterogeneous reaction; the surface morphology changes from loose porous structure formed by physical adsorption of large round particles to uniform and dense thin film grown by liquid phase epitaxy, the grain shape changes from willow catkins to granular, the grain size gradually increases, the particle size distribution is more uniform, and the film surface is smoother; the electrical parameters such as EQE, Voc,Jsc, FF and Rs are optimized, and the photoelectric conversion efficiency increases from 7.64% to 13.60%.