Abstract: The discontinuity of rectangular waveguide and coaxial line is theoretical analyzed by use of the mode-matching method with high precision. The Ka-band rectangular waveguide- coaxial transition is designed using synthetic method with impedance transitions as well. Finally, the performance of the method is modeled using electromagnetic field simulation and verified with the Ansoft HFSS software. The result shows that the performance reaches taller inland level and the engineering demand, that is, in the bandwidth between 26.5Ghz and 40GHz, the insertion loss is no more than 1.0dB, VSWR is no more than 1.2. It is known that the method in this paper is effective by comparing simulated result with performance result.