Based on the smooth cylindrical waveguide structure, an novel polarization converter for high-order TEmn mode is presented. Using the general coupled wave theory and numerical optimization method, a polarization converter for 94GHz TE5,1 mode is studied. The proposed polarizer, consisting of transition section and phase shift section, features structurally simple and easy construction. From 88GHz to 98GHz, the maximum axial ratio of the polarizer is less than 1.18. The performance of polarizer was veritfied by comparing the ideal radiation field and the measured one. The research work provides a physics model for designing low-order waveguide mode polarizer with high conversion effciency.