Abstract: To realize RF phase control for the purpose of coherent power combination, an X-band high-power relativistic backward wave oscillator (RBWO) with an input signal is presented. In the device, an independent input cavity is introduced before the slow wave structure (SWS) to reduce the leakage power from the input port and modulate the electron beam. The beam is intensively bunched when it enters the SWS and gives out its kinetic energy into RF radiation. PIC simulation shows that an input power of 6 kW is sufficient to control the phase of 2.5 GW output microwave.