In order to study the effect which the lateral and axial defocus of pinhole have on diffraction intensity and reference wavefront quality in point diffraction interferometer, a mathematical model of convergent beam diffracted by a pinhole is established based on Fresnel diffraction theory. A novel method is proposed which combines the diffraction intensity with the peak value of diffraction wavefront error to analyze the wavefront diffracted by pinhole. F number of the converging beam and diameter of the pinhole are also taken into consideration. The results show that either diffraction intensity or reference wavefront error varies in inverse proportion to the F number of converging beam. The amount of lateral defocus should be less than the difference value of the radius of Airy disk and the pinhole. The acceptable value of lateral defocus is directly proportional to the wavelength and F number while being inversely proportional to the pinhole′s diameter. With the beam F number of 10 and the lateral defocus ranging from 0 to 35 mm, the pinhole with a diameter less than 1 mm can satisfy the requirement that the peak valley (PV) value of wavefront error should be less than 0.1 λ . The reference wavefront intensity can be changed by adjusting the axis defocus. The smaller the diameter of the hole is, the more sensitive the change is. A pinhole with a diameter less than the radius of Airy disk can make sure that the peak value of diffraction wavefront error is less than 0.01 λ and the test wavefront intensity should be controlled around one tenth of the effective diffraction intensity of the pinhole. The simulation results provide a theoretical basis and data reference for the design of point diffraction interferometer and its accuracy analysis.