Based on the Kirhhoff diffraction theory, we Studied the filter hole of the point diffraction interferometer (PDI). Through an lysis of the results, we find that the position of the hole influence greatly the quality of the reference wave pro uced by it, though its dimension may be smaller than the Airy spot. On the receiving plane with a great phase deviation of the reference wave, there always exists a gre t weadening in its intensity. An interferogram of a PDI showing the phenomena is presented.