Abstract: The relativistic magnetron is a crossed-field device and plays an important role in the high power microwave technology and radar system. Recently, researchers realized mode switching in relativistic magnetron between neighbour modes as well as different longitudinal operation modes in the same modes with several hundred kilowatts RF input signal. Unfortunately, the RF input power is assumed to already power into the relativistic magnetron cavity, though in reality not considering such work. A novel RF input technology is proposed in this article, using a rod antenna to couple the input RF signal from the input sector waveguide into the relativistic magnetron cavity. In the PIC simulation with UNIPIC code, by adjusting the antenna position and the antenna configuration, the coupling efficiency between the antenna and input sector waveguide can be optimized. This new method can be used in power and energy input for mode switching, and the simulation results can be used as references for RF signal input in future experiment.