Abstract: Cat-eye effect occurs when a parallel beam or a laser beam illuminates a focal plane imaging system in the direction of optical axis, where the orthogonality and defocus of the detector in relation with the optical axis and focal plane affects directly the property of the reflected light beam, further affects the performance of cat-eye effect based detecting system. The relation between the defocus amount and the divergence of reflected beam was investigated through both theoretical analysis and numerical calculation.When the defocus is 0.16mm, the minimum divergence angle is 0.019 mrad. Furthermore, an experimental system was set up to test the defocus and divergence, When the defocus is 0.15 mm, the minimum divergence angle is 0.25 mrad,indicating the test result agrees well with the calculation. The conclusions drawn from the investigation are as follows：1） the impact of negative defocus on the divergence of reflected beam is greater than that of positive defocus； 2）the divergence angle of the cat-eye reflected beam increases virtually along with the increasing of absolute defocus, but the function diagram of the divergence is not symmetric in shape with respect to the zero defocus point, rather, a unique minimal divergence exists at a certain positive defocus point.