Abstract: Using the 4-D code for laser propagation in the atmosphere, uplink for the relay mirror system was simulated. Under H-V5/7 turbulence model, the relationship between launching power and power coupling efficiency between the receiver at the relay platform and the transmitting telescope on the ground, received power and peak power density of far-field at the relay platform was investigated. The result shows that the received power at the relay platform in creased with the increase of coupling efficiency and beam quality of the received laser. A critical distortion number NDc, which when the distortion number exceeded the peak power of the far-field would descend, also existed. The critical value with different turbulence strength for uplink propagation of relay mirror system was further studied, and a fitting result was obtained and could be expressed as NDc=22.36e-0.26(D/r0)+36.87.