Abstract: The operation efficiency of the even-period relativistic backward wave oscillator (RBWO) with slow-wave structure was low. In order to obtain a higher efficiency, we design an X-band uneven-period RBWO. The simulation demonstrates that when the voltage is 719kV, the current is 10.2kA and the magnetic field strength is 3.0T, a microwave with power of 2.81GW is generated, with an conversion efficiency of 38.3%. The frequency of the output microwave is 9.04GHz, and the output mode appears as TM01. As shown in the results, the uneven-period slow wave structure can promote the operation efficiency of the devices. Moreover, we analyze how the electron-beam voltage influences the output power, efficiency and the operation frequency.