In this paper, the input coupler for W-band gyro-TWT is designed to meet the needs of the signal transmission and mode conversion. The input coupler is potential device to be the essential section determining the bandwidth of the whole gyro-TWT. The theoretical analysis of the input coupler of the W-band TE₀₂ mode gyro-TWT is carried out. It is pointed out that one factor affecting the transmission loss of the main mode is that the rise of the miscellaneous modes reduces the transmission coefficient of the main mode, and simulation is done to suppress the miscellaneous modes by optimizing the sizes of the coupling hole, reducing the loss from 3.9 dB to 0.8 dB. According to the simulation sizes processing and cold testing, the final input coupler with 3 dB bandwidth of 7.9 GHz meets the design requirements.