Based on the coupled wave theory, we have designed the TE01–HE11 high efficiency and high-power mode converter at 35 GHz through the method of numerical simulation, where the TM11 mode is used as the intermediate mode. The ohmic losses due to the metal walls are taken into account in the calculations, and rematching phase techniques are used to increase the conversion efficiency. Modeling analysis is performed using commercial CST Microwave Studio software, and the cold test is performed. Experimental results verify the accuracy of numerical simulation.