The quality of the reference wave front in phase shifting point diffraction interferometer depends on the pinhole diameter, roundness and thickness, where the first one is the most critical factor. The requirement for pinhole diameter should be known before the manufacture in actual maching. Three-dimensional pinhole diffraction is calculated based on the vector diffraction theory. How the pinhole diameter affects the diffracted wave-front quality is analyzed under the uniform incident light with TE and TM polarization. The appearance of the astigmatism and coma in the wave front is brought about by the linear polarization of incident light. The calculation and analysis show that in order to obtain reference wave front with 0.1 numerical aperture (NA), the root mean square of (RMS) wave-front error below 0.005 λ (λ=13.55 nm), together with 0.4 intensity uniformity, for a 90 nm thickness pinhole, the diameter of 70 nm is suitable.