This paper mainly discusses about 90GHz—100GHZ pillbox window which window chip used sapphire for TWT.We simulated and optimized it by HFSS,and the result shows that the VSWR of the two kinds of windows are less than 1.27,and the second kind of the window used bigger cylindrical wave guide which can make it easier to processing than the first kind.We fabricated the first kind of window,and tested it.