Synthetic aperture technique can be used to improve the reconstruction image resolution of digital hologram. In this paper, a synthetic aperture digital holographic method is proposed. In the method, the recording of synthetic aperture digital hologram is carried out by means of object movement in one direction, and the reconstruction is achieved by cross-correlation synthesis technique. Compared to the digital holography without synthetic aperture technique, only a one-dimension movable bracket is added in the experimental setup. The recorded object fixed on the movable bracket is placed in Mach-Zehnder interferometer and moved along horizontal direction. For the purpose of aperture synthesis reconstructing, four sub-digital holograms have been recorded by a CCD camera. While recording, superposition area of about half pixels of single digital hologram size should be assured between the adjacent holograms in order to ensure that the sub-digital holograms are synthesized accurately. The experimental result shows that every horizontal line of the reconstruction image is distinct in the synthetic aperture digital hologram, but the transverse is unidentifiable in the images reconstructed from the single digital holograms. It can be seen obviously that compared to sub-digital hologram the reconstruction image resolution of synthetic aperture digital hologram is improved.