This paper introduces the fundamental principle of the recording and numerical reconstruction of the synthetic aperture digital holography, and proposes two numerical reconstructed methods: one is the numerical reconstructed complex amplitude superposition or intensity superposition from the single reference beam sub hologram, the other one is the numerical reconstructed complex amplitude superposition or intensity superposition of the multi reference beams sub hologram. The experimental research results show: Synthetic aperture technique can improve the resolution of digital holography evidently. Comparing with the conventional numerical reconstructed method of direct combined sub-hologram, the proposed two kinds of numerical reconstructed methods are the same in improving the resolution of reconstructing image of digital holography, but the difficulty of recording and reconstruction of synthetic aperture digital holography by the intensity superposition method is decreased.