Abstract: On the basis of total internal reflection edge illumination, a method can be used to identify the damage on front or rear surface of the thin DKDP crystal by the birefringence of DKDP crystal. When an ultraviolet ray incidented upon a DKDP crystal whose thickness was 11 mm, a displacement of 254.738 m(theoretical value) between o-ray and e-ray on the exit surface will be caused due to the birefringence of DKDP crystal. The damage located on the rear surface of DKDP had double-images, which could be modulated by polarizer placed in front of CCD camera. The damage located on the front surface of DKDP had a single-image, which couldn't be modulated by polarizer. The method had the ability of avoiding extracting the information of a damage repeatedly, and improving the accuracy of damage identification. The experiments demonstrate that the method can distinguish the damage on front or rear surface of the thin DKDP crystal whose thickness is 11 mm.