The coaxial plate-inserted mode converter is a novel high-power microwave TEM-TE₁₁mode converter. The mode conversion processes of the mode converter were reviewed, and the reflective characteristics were studied by analyzing the waveguide discontinuousness. It is revealed that, the reflection modes of the mode converter are TEM mode and TE₁₁mode, the reflection TE₁₁ mode can be canceled by choosing the right radii of the coaxial waveguide, and the reflection TEM mode can be diminished by setting matching poles and properly designing the cone at the end of inner conductor.