The invention discloses a compact overmoded waveguide elbow suitable for a TE<1, 1>-mode gyrotron traveling wave tube, belonging to the field of high-power microwave transmission devices. The elbow includes a 90-degree curved rectangular waveguide and two rectangular-circular waveguide transition structures connected with two ends of the curved rectangular waveguide, internal cavity connection parts of the three structures being smooth transitions. After circularly polarized TE<1, 1>-mode wave is transited through the circular-rectangular waveguide, speeds of TE<1, 0>-mode guided wave and TE<0, 1>-mode guided wave generated in the orthogonal polarization directions are different due to the difference of the long and wide sides of the rectangular waveguide, and thus there is a phase difference; and after the waves are turned through the 90-degree curved rectangular waveguide subsequently, the phase difference is increased further. At an output end, the phase of an output beam is finally adjusted by the rectangular-circular waveguide transition section according to the output requirements. The elbow of the invention can realize the beam output of specific polarization states according to the application demands while guiding the beam to turn at high efficiency, and the whole elbow structure is simple and easy to process.