Abstract: This work focused on the metal posts' scattering calculation and design in coaxial waveguide (CWG), which are useful in many high-power microwave devices. The calculation model was constructed by equating CWG with rectangular waveguide. The scattering formula was deduced by Integral Equation Method, which was validated by Finite Element Method. The calculation and experiment results show that the posts will excite a series of TE(mn)1 modes in CWG when TEM mode is inputted. While the number of the posts is greater than the ratio of the CWG's average perimeter to wavelength, all of the TE(mn)1 modes are evanescent, with only TEM mode being outputted. The reflection coefficient of TEM mode increases with number or radius of the posts, and is in inverse proportion to frequency.