Abstract: A high-resolution and long working distance digital holographic microscopy is proposed based on synthetic aperture method. A series of digital holograms are successively recorded, which cover different spatial frequency ranges scattered from the object, with different tilted object illuminations. Subsequently, the intensity image is numerically reconstructed and magnified for each digital hologram. Finally, a synthetic image with the resolution improved and speckle noise suppressed is obtained by synthesizing the reconstructed images. The theory and experimental results show that the proposed method can be easily used in in-situ microscopy at a long working distance.