The quality of CVD diamond window has a direct influence on the stable and safe operation of ECRH system. Under the condition of the CVD diamond window by HE11 mode, based on CVD diamond line of microwave window material, thermodynamic finite element analysis software is used to analyse the output window for steady-state heat flow and solid coupling analysis and structure statics analysis, obtained the best cooling water flow rate, make the window of CVD diamond center minimum temperature and thermal stress; The CVD diamond window sheet with a radius of 38mm and a thickness of 1.36mm was obtained, and the maximum CW power of which was about 1MW. Then, the dielectric properties of CVD diamond window were measured by free-space method, and the dielectric parameters were better when the central frequency was 140GHz. It provides a reliable theoretical basis for ECRH system CVD diamond window high power test.