Abstract: In the circular symmetrical magnetic field, the annular electron beam impacts on the cylindrical relativistic backward-wave oscillator (RBWO), in which the energy of the electrons are deposited. The model of the electron energy deposition is built with Monte Carlo code FLUKA, and the distribution of the electron energy deposition is studied. The motion model for the backscattering electron is built and the electron trajectory in magnetic field is simulated. The method for approximating the circular symmetrical magnetic field is developed. Several conclusions are made as follows: with the magnetic field enhanced, the maximal energy deposition density becomes larger, and the position of the backscattering electron is nearer to the incident electrons region and a new energy deposition peak may occur. When the magnetic field is strong enough, the unidirectional magnetic field could be applied to the calculation of the energy deposition distribution under circular symmetrical magnetic field.