Abstract: Beam scanning of large aperture reflector antenna on satellite may increase the moment of inertia and mechanicalcomplicity of the system by movement of the whole antenna system. A new method was analyzed to make antenna’s main beam scanthe desired direction through the movement of sub-reflector with main reflector and feed system fixed. In a Cassegrain antennasystem, when the sub-reflector locates the optimum location, the antenna performance has the lowest degradation. Ray-traceoptimization was used to calculate the optimum sub-reflector location. Detail design was given and both simulation and experimentresults demonstrated the computation method