Strong diffraction and scattering effects can be caused by micro or sub-micro scaled surface defects of optical components. The distribution of scattering light on the image plane can not be explained simply by geometrical optical theories. The electromagnetic model of triangle-shaped and rectangular surface defects is built by using finite difference time domain (FDTD) method. The distributions of scattering light in the near-field region and on the image plane are simulated. The scattering imaging experiment of sample defects is also performed. The sample defects are rectangular and fabricated by electron beam exposure and ion beam etching. The gray distributions in dark-field images are compared with intensity distribution of scattering light on the image plane and show a good consistency. The results provide significant theoretical references for defect calibration in optical manufacturing and testing.