On the basis of coherent image theory,starting with the analysis of a point spread function of digital holography,the influence factor of the reconstructed image quality in digital holography was studied.The main analysis was that,for a certain CCD and the recording wavelength,the transfer function was influenced by the recording distance,and the reconstructed image quality was also investigated.It comes to a conclusion that the object should be placed as close as possible to the CCD target in the digital recording of hologram when the sampling theorem is fulfilled.The mathematical description of the minimum allowable recording distance in digital holography was given,and the relevant experimental results were also presented.The experimental results show the validity of theoretical anlaysis.