Bubbles， impurities， scratches and other defects in the inner optical elements of the lens will seriously affect the imaging performance of the optical system of the security lens. A new method for detecting lens defects in security lens is presented in this paper. In the dark environment， backlight and ring light are used to strike the test lens respectively， and then the object square telephoto lens is used to image the high-resolution CMOS detector to take the head-on defect images of the lens to be tested. The results of the test show that the accuracy of the system is up to 90%， and it has a certain guiding role in the research on the automatic detection of industrial security lenses and the detection of optical instruments.