Abstract: In relativistic backward-wave oscillator(RBWO), electrons bomb on the collector surface under the guiding of magnetic field at some incident angle after the beam-wave interaction. Restriction from guiding magnetic field to electrons was analyzed theoretically, and condition for backscattering electrons escaping from the collector surface was derived. Using Monte Carlo method, energy deposition of electrons was calculated. It was found that restriction from guiding magnetic field to electrons moving inside materials was quite weak, but was extremely strong out of materials. Most of the backscattering electrons strike again on the collector surface due to Larmor precession. Electrons escaping enter into beam-wave interaction area, reducing the efficiency of energy transformation and output of microwave. When enlarging the incident angle of spent electrons, reduction of beam current density and re-bombardment of backscattering electrons result in the drop of the maximum energy deposition density and improvement of bombard-resistance of collectors.