To satisfy the application requirements of sapphire output window for the 170 GHz megawatt-class gyrotron short-pulsed experiment, by calculating the characteristic of reflection and absorbing of Gaussian beam transmitted through sapphire window, a sapphire window unit is designed with low reflection and low loss power. The low reflectance is verified by vector network analyzer that the Voltage Standing Wave Ratio(VSWR)<1.1. The maximum output average power threshold of the sapphire window has been calculated approximately to be 590 W by analyzing the damage mechanism. The 170 GHz gyrotron employing sapphire window is fabricated, and the power threshold calculated theoretically is verified, providing a reliable theoretical basis for the application of sapphire output window used in high power gyrotron.