The quality of the diffracted reference spherical wave in point diffraction interferometer (PDI) depends on the aberrations in the illumination objective lens and the quality and status of the pinhole. The pinhole diffraction of the visible light focused by the aberrated illumination objective lens is calculated based on the vector diffraction theory, the finite thickness and real conductivity of the pinhole are considered. The effect of illumination objective lens aberration on the quality of far-field diffracted wave front is analyzed. The best pinhole diameter of the PDI used to test the extreme ultraviolet lithography (EUVL) system and its element are determined. The calculation and analysis show that, when a system with NA of 0.3 is tested by PDI, the appropriate pinhole diameter is 800 nm, the RMS deviation of the diffracted wave front is 6.51×10-5λ, intensity uniformity is 0.812, when an optical element with NA of 0.3 is tested by PDI, the appropriate pinhole diameter is 500 nm, the RMS deviation of the diffracted wave front is 8.40×10-5λ, intensity uniformity is 0.664.