In this paper, we propose a method for deriving beam pattern masks to maximize the aperture efficiency of the offset reflector antenna. The beam pattern mask is designed to be proportional to the square of the distance from the feed to the reflector, and we obtain the optimum beam pattern using beam synthesis. As a result, employing the synthesized beam as the feed for the offset reflector yields an aperture efficiency of 81.6%, sidelobe level (SLL) of 36.7 dB, and an axial ratio (AR) of 2.30 dB. These results demonstrate that the proposed mask provides superior aperture efficiency, enhanced sidelobe suppression, and improved CP characteristics.