The hard edge aperture effect of cats eye optical system is studied by matrix decomposition and the hard edge aperture window function is expanded into the superposition of complex Gaussian functions. The reflection law of laser is analyzed by numerical calculation. The study reveals that the farther the waist of the incident beam is from the cats eye optical system, the greater the aperture effect of the optical system. Meanwhile, adjusting the proper amount of positive defocus can make the echo light intensity of cats eye optical system higher. Theoretical analysis and numerical calculation have certain reference significance for the study of cats eye system.