A beam switching system, beam switching method, and antenna device are provided. The beam switching system includes a selection circuit and a branch line coupler. The selection circuit includes an input port for receiving the input radio frequency signal and two output ports. The selection circuit selects at least one of its two output ports to output the output radio frequency signal. The branch line coupler includes two input ports respectively coupled to two output ports of the selection circuit and used to receive the output radio frequency signal, and two output ports respectively used to couple two antennas.