There are many advantages while reconstructing hologram in optal way, suchas the easy elimination of aberrations, noises, and nonlinearity etc. As to Shortwavelength holography (X-ray holography, electron .hologtaphy), resolution can begreatly improved by means of digital reconstruction. The theoretical formulae for digitalreconstruction of hologram are given. Reconstructions of in-line and off-axis hologramrespectively are brought out successfully from ideal holograms in digital way. All theresults are also analyzed.