The dimension of pinhole affects the error of diffraction wavefront, which is the critical element that determines the precision of point diffraction interferometer (PDI). The deviation of diffracted wavefront from an ideal sphere at visible wavelength, which is aroused by the dimension of the pinhole, is numerically analyzed based on the finite difference time domain (FDTD) method. The simulation results show that the peaktovalley (PV) value of diffraction wavefront error is less than 1 nm and the root mean square (RMS) value is less than 0.35 nm when the diameter of pinhole is 400 nm and numerical aperture (NA) is 0.65. The high precision of testing spherical surfaces with high NA is demonstrated. The analysis of pointdiffraction wavefront error with FDTD method provides theoretical basis for choosing the dimension of pinhole in experiments and tests with PDI.