Based on an ultrashort H-plane coupler and a cut-off structure, a high frequency structure of a W-band Periodically Cusped-Magnetic (PCM) field focused wideband sheet-beam Traveling-Wave Tube (TWT) is shortened by 27.7 mm. This approach allows the TWT to generate more than 200 W of power within a bandwidth of 13 GHz (90~103 GHz). The interaction between the electron beam and microwave in the ultrashort cut-off structure is investigated. The study shows that the ultrashort cut-off structure can mitigate the divergence of bunched electrons at the cut off structure when the first section of the Slow-Wave Structure (SWS) is long enough, thereby significantly enhancing the output power and bandwidth of the TWT. Therefore, long cut-off structures, high input signal voltages, and low electron gun voltages should be avoided when a wideband TWT is designed.