Abstract: High gain reflector antennas with flat top beams are used in the fields where high power uniform field is needed, such as satellite constellation tracking and long distance microwave energy transmission. In this paper, inspired by the multi-mode and mixed-mode horn antenna, the idea of multi-mode is innovatively introduced into the design of reflector antenna to realize the flat-top pattern. Firstly, a dual-mode reflector is designed, that is, an inverting mode field with a phase difference of 180° is introduced in the maximum radiation direction of the parabolic antenna, and the flat-top beam is synthesized from the two modes. Then, the general theory of multi-mode reflector antenna is summarized by extending the concept of dual mode. Different regions of the reflector excite multiple modes of signals and synthesize the desired shape pattern. In addition, the structures, modal variation and realization of multi-mode reflector antenna are introduced in detail, and a C-band high-gain reflector antenna with flat top pattern is successfully designed according to this theory. The work in this paper provides new theoretical and technical support for the development and application of traditional reflector antennas.