The invention discloses an X wave band over-mode relativistic klystron amplifier, which comprises an annular cathode, a resonant reflector, an input cavity, a first section of drift tube, a wave absorbing material, a buncher cavity, a second section of drift tube, an output cavity and a magnetic field coil, wherein the annular cathode is arranged at the most significant end of the structure and emits annular relativistic electron beams outwards under the action of high voltage pulse, the resonant reflector, the input cavity, the first section of drift tube, the wave absorbing material, the buncher cavity, the second section of drift tube and the output cavity are sequentially arranged at rear of the annular cathode, the magnetic field coil is installed at the periphery of the whole structure, the working mode of the resonant reflector, the input cavity, the buncher cavity and the output cavity is a TM02 mode, and the first section of drift tube and the second section of drift tube can transmit a TM01 mode. The X wave band over-mode relativistic klystron amplifier can produce high power X wave band microwave.