In the terahertz frequency band, the dimensions of microwave windows within traveling-wave tubes become very small, and transmission losses grow increasingly significant, severely impacting the output power of electromagnetic signals. To mitigate rapid attenuation of the signal generated by the TWT amplifier, an over-mode window has been proposed. The window disc dimensions of the over-mode window are larger than those of a standard pillbox window at the same frequency band, ensuring reliable window sealing. Utilizing a non-standard WR1.6 waveguide instead of a WR1 waveguide effectively reduces the attenuation of terahertz electromagnetic waves and enhances the circuit's power handling capacity. Test results at 1 THz demonstrate good matching for both types of over-mode windows, with bandwidths exceeding 20 GHz for return loss below −15 dB. The insertion loss within the band is below 5.5 dB. The overall electromagnetic performance is superior to that of the pillbox window-based transmission system. An over-mode window with a wider bandwidth has been optimized and experiments are to be conducted.