Abstract: Using the equivalent circuit theory, we design a pill-box output window for Q-band high power gyrotron traveling wave tube (gyro-TWT) with a thickness of 1.32 mm. Through inserting the inductive diaphragms and changing the shape of the window, the designed pill-box output window can withstand an average power of 25 kW, the relative bandwidth is 14%, and its thickness is changed to 1.7 mm. This paper uses a high frequency software HFSS combined with a new method of collaborative simulation of finite element analysis software ANSYS to simulate the thermal characteristics of the pill-box output window. It shows that the new-type of pill-box can withstand an average power capacity of 25 kW at the bandwidth, and the temperature difference between center and edge of the window is 66 ℃, while the tolerance temperature is 158 ℃ of the ceramic window. Thus, it verifies the rationality of the new pill-box output window for Q-band high power gyro-TWT.