Abstract: The relay mirror system is a new optical system which can effectively decrease the atmospheric aberrations and enlarger the laser attack scope. The standard used to judge the advantage of beam propagation through a relay mirror system and the method used to calculate its advantageous application scope are key contents in the relay mirror technology. In the present paper, model of beam propagation through a relay mirror system was established, the equivalent Fresnel number of beam propagation through a relay mirror system was ratiocinated in detail, the performance evaluation factor used in the relay mirror system was defined, the relationship between the factor and the equivalent Fresnel number was analyzed, and the method used to calculate the advantageous application scope of a relay mirror system was analyzed. Under the Hufnagel-Valley 5/7 turbulent model, performance of a 30 km relay mirror system with 3.8 m ground-based laser source was calculated. Results show that the relay mirror system can improve laser attack effect to long-distance and high-altitude targets, and enlarger the laser attack scope.