The diameter of pinhole or fiber is a critical element that affects the precision of point diffraction interferometer (PDI). The dimension of the aperture is determined by variational accuracy of practical measurement. Based on the scalar diffraction theory, the deviation between diffraction wavefront and sphericity aroused by the dimensions of the aperture is simulated and analyzed. The results show the wavefront error of the optical system is about 0.07 nm peak-vally value (PV) when the numerical aperture (NA) is 0.3 and the aperture diameter is 2.5 μm. The method and results of simulation analysis provide theory and data reference for choosing the dimension of aperture along with the analysis or design of PDI for various testing tasks of accuracy requirement.