Abstract: In order to increase the power capacity and satisfy the demand of the vacuum sealing of microwave system , a X-band new type circular waveguide was designed based on the coupling wave theory and the four-port network, and it is verified by electromagnetic field simulation software . The rectangular-square-circular waveguide transformer is adopted in this novel output window, and the small radius circular waveguide is extended to a circular waveguide with a larger radius. Results show that, under the center frequency of 9.37GHz, this circular waveguide widow’s standing-wave ratio is 1.03, and power capacity attain about 18MW.