Abstract: The dispersion characteristics of slow wave structure(SWS) and the beam-wave interaction theory are developed for klystron-like relativistic backward wave oscillator. The dispersion characteristics show that the operation mode of the device is TM01 mode, near point, and the maximum growth rate is relatively small, which is partly due to the larger span between the electron beam and SWS and will result in a larger saturation time. The frequency predicated by the dispersion characteristics is very close to that obtained in previous particle-in-cell simulation. In the beam-wave interaction theory, the 0th space harmonic of forward wave, the -1st space harmonic of backward wave, and the 1st-order beam space-charge wave are taken into account. The resonant reflector provides pre-modulation for beam current and electron energy, and the modulation cavity and extraction cavity introduce larger changes in coupling impedance and phase velocity. Both steady state and unsteady state calculations show that beam-wave conversion efficiency of above 40% can be obtained.