Abstract: This paper reports the simulation investigation of an X-band relativistic magnetron with diffraction output, which contains an 18 resonant cavity system. The operating performance of the device is improved by optimizing the structural parameters and reasonable selection of magnetic field strength and electric field intensity using software of particle simulation and high-frequency field analysis. Simulation results show that an output microwave which has an output power of 1.09GW, power conversion efficiency of 22% and frequency of 9.59GHz ,with an operating voltage of 350kV and a magnetic field of 0.41T.