Model of a relay mirror system was established, and the relationships between system damage effect and the uplink telescope aperture in a 30km-altitude relay mirror system were calculated. Results show that, with the increase of the uplink telescope aperture, the system damage effect increases significantly when the uplink telescope aperture is less than 0.42m; the system damage effect decreases sharply when the uplink telescope aperture is in 0.42-0.54m; the system damage effect increases when the uplink telescope aperture is greater than 0.54m，while gradient of the increasing reduces; the system damage effect tends to be a stable value when the telescope aperture is greater than a certain size. Reasons for the steep fall in 0.42-0.54m are analyzed, and it is demonstrated that the intensity non-uniformity of the received beam causes the reduction of the new source beam quality and the steep fall of the system damage effect. The results of the study can provide reference for the relay mirror system.