Abstract: With the development of high power klystron, the energy transmission window not only needs to meet the requirements of good standing wave matching, thermal reliability and etc. , but also needs to keep the structural reliability under high pressure. A new output window of high power klystron is introduced in this paper. Different from the conventional waveguide window, this output window shapes like “tongue"and adopts ceramics material. It not only has the good standing wave matching in the frequency range of 2.05 ~2.7 GHz (relative bandwidth 26.8%), and the reliability at 20 kW transmitting power, but also achieves the structural reliability under the high pressure (0.9 MPa). The development of this output window solves the problem that klystron energy transmission window is damaged by high pressure in high power system, and provides technical support for the application of high power klystron in high power equipment.