A sort of universal theoretical analysis method for multilayer dielectric window was presented by using the mode-matching technique to build scattering matrix. The analysis of output window for high power millimeter-wave gyro-TWT was prosecuted. Based on academic analysis and numerical calculations , the primitive structure and geometric parameters of the output window had been obtained. Afterward, HFSS code is used to simulate accurately and revise approximate analysis. Through a large number of thermodynamic analysis and optimization design, the wide-band output windows of Ka-band , with average power capability 50 kW and band width about 3.6 GHz when S11 parameters less than -20 dB , were attained. The cold test indicates that the results of design are in agreement with the results of cold test.