Abstract: The paper introduces how to use high frequency structure simulator (HFSS) to establish a physical model of wave-guide window and calculate the distribution, magnitude and direction of electric field inside wave-guide window. In addition, some of RF parameters of wave-guide window such as frequency, bandwidth and VSWR are also calculated. All of these provide a thoroughly theoretic basis for how to improve the wave-guide window and ultimately a new-type of wave-guide window for transmitting peak power of 5 MW, average power of 45 kW is well designed.