太赫兹波在基模波导中传输的欧姆损耗很大，因此在传输太赫兹波时经常采用过模传输的方式来降低传输损耗。但采用过模传输，会引起波导中传输模式的变化，因此如何保证传输中的模式稳定是过模传输中的重要问题。本文采用仿真与实验的方法，对220 GHz 圆传输波导中的圆波导半径渐变情况下的模式稳定性进行了研究。结果表明，适当延长渐变波导长度，可以抑制模式耦合，保持单模传输。

Because of the huge ohmic losses of the fundamental mode waveguide in terahertz wave, over-mode transmission line is often adopted to reduce the transmission loss. Nevertheless, the waveguide mode would be changed by using over-mode transmission line. How to ensure the stability of the mode is an important issue. By using simulation and experimental methods, the stability of the circular waveguide at 220 GHz is researched. The results show that prolonging taper waveguide properly can inhibit the mode change and maintain single mode.