Abstract: An iterative method based on coupled mode theory is proposed in this paper, also structural controlling and multi-frequency design techniques are proposed to satisfy the structural property and frequency response property requirements. The iteration method can be used to design many kinds of waveguide components. A TE01-TE11 mode converter is designed, the working frequency of which is 35 GHz, and the radius is 13.6 mm. The multi-frequency technique is utilized. The bandwidth reaches 13.6%, which is the 2.3 times of conventional method.