The influence of illumination parameters on the quality of reconstructed images in digital reconstruction of an off-axis Fresnel hologram is analyzed. A new algorithm is proposed to extract the optimal illuminating orientation according to the spatial spectra of the hologram. And a digital reconstruction program for off-axis Fresnel holograms is designed in MATLAB, in which the optimal illumination beam is obtained automatically by analyzing the spatial spectra of the hologram,and the exact image plane is found by an auto-focusing algorithm. The pixel size of the image sensor can be also calculated by use of the parameters of the object distances measured in recording and reconstructing process at the same time. The theoretical analysis and experimental results are aualysed.