Abstract: Aerospace Relay Mirror System (ARMS) as a new conceptional weapon used in reflection of High Energy Laser (HEL) in space is studied. It extends the attacking range of Ground Based Laser (GBL) weapons enormously. Relevant researches have been carried out for many years. The structure and operating theory of the entire system consists of ground-based high energy laser weapon and space-based relay mirror. The main work of this study was focused on the analysis of relay mirror energy character to deduce the relationship among the attacking range of weapon and diameter of ARMS and target irradiance. Calculation examples are also given. The result further proves the strong points and feasibility of ARMS in principles. It is summarized that more importance should be attached to ARMS development. The results given in this paper can be the reference of actual ARMS system to some extends.