Abstract: The influence of retro-reflect modulation reflected optical characteristics on the retro-reflect modulation structure parameters and receiving distance were studied by establishing off-focus structure equivalent model of retro-reflect modulation, and the theory of retro-reflect modulation model with geometrical optics method was analyzed. Some expressions related to the structure was given, numerical calculation and experiment were completed. Results show that the retro-reflect modulation structure parameters and receiving distance affect the variations of modulation system reflection power boviously and regularly. Moreover, reducing the lens focal length, increasing the lens aperture and increasing the receiving distance can increase the rate of change, and make contribution to improve the modulation effect. And under normal circumstances, the divergence of the reflected beams is much smaller when the mirror in the focal plane moves forward out of focus.