Input/output window is one of the important parts of the Traveling Wave Tube(TWT), which directly influences the capability of the TWT. The broadband window is one of the important development directions. Broadband window for 140 GHz TWT is designed, fabricated and tested. Test results show that the S11 parameter is less than -13.6 dB, and the S21 parameter is no more than -0.8 dB when the broadband window is between 130 GHz and 150 GHz, which verifies the feasibility of broadband window design and lays a foundation for the development of broadband TWT.