In this paper, a reflectarray based dual-beam gain-reconfigurable structure is proposed for dynamically controlling the gain of each beam. In this structure, two phased-array feeds are illuminating one shared reflectarray aperture. By modulating the excitation conditions for each phased-array feed and compensation phase distribution on the aperture, dual beams with different gains can be realized simultaneously, which can be useful for fully terminal coverage for the base station antennas of mobile communications. Two sets of beam states have been chosen to validate the novel structure. From the simulation results, a gain of 20.9dB is obtained for equal beam-state resulting in 39.1% aperture efficiency. For the other beam-state, two beams with 22.2dB and 18.4dB gains are achieved. The corresponding aperture efficiency are 40.4% and 31.4%, respectively.