The model for uplink propagation for the relay mirror systems was established, and the effect of aperture matching on uplink propagation for relay mirror systems was simulated and analyzed by the 4-D code for laser propagation in atmosphere. The far-field peak power density of the received beam whose distorted phase was completely removed was selected as the evaluation function of the uplink propagation process. The optimal focal lengths of different transmitting apertures were picked out based on the calculation in vacuum, and the uplink propagation in the atmosphere for different transmitting apertures with their respective optimal focal length was simulated. Results show that with a certain receiver aperture, the optimal focal length decreases while the transmitter diameter increases. The critical launching power increases with the increase of transmitter diameter. When the launching power is small, the change in transmitter diameter imposes little effect on the performance, but when the launching power is high enough, performance of the relay mirror system is improved significantly with the increase of transmitter diameter.