To realize the high-purity TE₀₁ mode input of the Ka-band cyclotron traveling wave tube, the two-stage power divider TE₀₁ input coupler is improved by adding a filter structure at the output port, and the transmission efficiency of the spurious mode is reduced from the original average of 7% to less than 2%. The main body of the structure is a cascading two-stage Y-type power divider network, which can efficiently realize the conversion of rectangular waveguide TE₁₀ mode to circular waveguide TE₀₁ mode. Based on the theoretical analysis of transmission performance of power divider network based on transmission line theory, and with the help of 3D electromagnetic simulation software, several optimization iterations are carried out, and finally a wideband TE₀₁ input coupler with mode conversion efficiency greater than 99% near the frequency of 31 GHz is obtained, the input coupling structure has a -0.1 dB insertion loss bandwidth of 5 GHz, and the average mode conversion efficiency in the effective frequency band is as high as 98.6%, the mode purity is 99% and the return loss is less than -15 dB. Back-to-back cold test experiments were carried out on the device using vector network analyzer, and the results show that the in-band attenuation is about 0.5 dB, which deviates little from the simulation calculation result, which meets the actual engineering requirements.