Two main determining factors,which limit the quality of the reconstructed image from the microscopic digital hologram by employing the angular spectrum method,are discussed. First are the shape and the size of the spatial filter selected. The corresponding selection principle of the spatial filter is presented,and the experimental result shows that the manual spatial filter exactly selected according to the distribution of the spectrum of the virtual image should be employed to obtain the optimal reconstructed phase image. Second is the phase tilt aberration of the reconstructed image when the specimen is put between two glasses. It is caused by the CCD plane or the sample plane being not perpendicular to the propagation axes. The correction method by the image processing is presented. The theoretical and the experimental result of an onion specimen are shown.