The complex amplitude distribution of diffraction light was analyzed by stationary phase integral method, when the point light source was passing through the system composed of two parallel placed transmission gratings with finite width. When the bi-grating imaging condition was satisfied, the comparison between the diffraction image of finite-width bi-grating and that of infinitely wide bi-grating was conducted. It is concluded that the position of bi-grating diffraction image has no effects from the finite-width grating;however,the size of diffraction image is affected by the relative distance and diffraction orders between two gratings. The size of diffraction image was calculated, which was enlarged by 1.22 times, when the condition of bi-grating imaging and some experimental parameters were satisfied. Furthermore,the study on the color of white light ordinary transmission hologram was done based on its principles.