Abstract: In order to accurately measure the coupling degree of the circular waveguide coupler, the influence of TE₁₁ and TE₂₁ modes on the coupling degree of the circular waveguide TM₀₁ mode converter is studied in this paper. According to the characteristics of TM₀₁, TE₁₁ and TE₂₁ mode field distribution in a circular waveguide, the influence of TE₁₁ or TE₂₁ mode content on coupling degree dispersion is studied by CST simulation. The influence of the relative angle between two mode converters on the mode in the coupler and the coupling degree dispersion is studied by simulation and experimental measurement. The results show that the dispersion of coupling degree increases with the increase of TE₁₁ or TE₂₁ mode content in couplers. When the coupling degree is measured with a rectangular TE₁₀ to circular waveguide TM₀₁ mode converter, the dispersion is large, and a small amount of TE₂₁ mode will appear in the coupler when the relative angle between the two mode converters is 0º or 180º, while a small amount of TE₁₁ mode will appear in the coupler when the relative angle is 90º or 270º. When the coupling degree is measured by the coaxial TEM to circular waveguide TM₀₁ mode converter, there is almost no TE₁₁ mode and TE₂₁ mode in the coupler, and the coupling degree is less dispersed. Comparing the simulation and experiment results of two different types of circular waveguide TM₀₁ mode converters in coupling degree measurement, it is concluded that using the average coupling degree can better eliminate the coupling degree dispersion and reflect the real coupling degree of couplers. The research provides a new test method and basis for accurately calibrating the coupling degree of couplers.