The invention belongs to the field of masers and relates to a relativistic backward wave oscillator of a direct circular polarization TE11 mode. The relativistic backward wave oscillator comprises an annular cathode, angular partition double-pre-modulation chambers, angular partition slow wave structures, an output waveguide and a magnetic field coil. The annular cathode is at the most front end of the structure and emits an annular relativistic electron beam under the effect of high voltage pulse. The angular partition double-pre-modulation chambers, the angular partition slow wave structures, and the output waveguide are orderly arranged behind the annular cathode. The magnetic field coil is at the periphery of the annular cathode, the angular partition double-pre-modulation chambers, and the angular partition slow wave structures. The angular partition double-pre-modulation chambers and the slow wave structures are employed by the invention, two linearly polarized TE11 modes with the same frequency, similar amplitude and orthogonal polarization can be generated in two groups of structure. Through the offset of d/4 of two groups of the double-pre-modulation chambers and the slow wave structures in an axial position, thus a 90-degree phase difference is introduced between the linearly polarized TE11 modes with orthogonal polarization, and the circular polarization TE11 mode is obtained in an output waveguide.