High quality spherical wave, which is generated by the pinhole diffraction, is the core for the calibration of the Shack-Hartmann wavefront sensor (SHWS) with high-accuracy. The diffraction of the tiny pinhole under the illumination of the excimer laser with wavelength of 193 nm is calculated based on finite-difference timedomain (FDTD) method. The finite thickness and real conductivity of the pinhole are considered. The effect of numerical aperture (NA) of the illumination objective lens on the quality of the wavefront diffracted by the tiny pinhole is analyzed. The diameter of the tiny pinhole and the NA of the illumination objective lens needed to calibrate the SHWS with high accuracy are determined. The calculation and analysis show that, to obtain the wavefront whose quality meets the requirement of wavefront error measurement by using SHWS method with nanometer accuracy, the diameter of the tiny pinhole should be 200 nm and the NA of the illumination objective lens should be in the range of 0.6 to 0.75. In this situation, the RMS deviation of the wavefront diffracted by the tiny pinhole is 3.50×10-3 λ ; the intensity uniformity is 0.10; the transmission of the tiny pinhole is about 0.15.