A new expression of partially coherent light is derived. The formula of the partially coherent optical field is consisted of two terms: spatially coherent part propagated by angular spectrum and incoherent part based on the optical transfer function. Both parts are related to coherent function (degree of coherence). Then a design method for computer generated hologram based on the theory of partially coherent light is proposed. The formation process of CGH is divided into angular spectrum and optical transfer function. Both numerical simulations and optical experiments are done to verify that speckle contrast of holographic reconstructed images is improved without sacrificing in PSNR compared to traditional method. The experimental results show that the speckle contrasts are improved more than 46%, and the image quality is obviously improved. This method can be applied on three-dimensional holographic display, beam shaping and other wave-front modulation techniques.