With the rapid development of THz technology, the waveguide attenuator used in THz system has also become a hotspot. The THz waveguide attenuator can accurately attenuate and control the power transmission, which shows special significance and plays an irreplaceable role in solving the problems of loss, radiation and interference. The current waveguide attenuator would destroy the wave guide transmission line and result in the RF leakage because it places the attenuation plate paralleling to the electric field. Based on the principle of the absorbent waveguide attenuator, a waveguide attenuator with the attenuation plate perpendicular to the electric field is proposed. The attenuation plate is attached to the inner wall of the waveguide. The waveguide attenuator is realized and optimized by using the HFSS software and changing the shape, position and other parameters of the attenuation plate. The return loss is less than -27.5 dB in 110-170 GHz, and the attenuator accuracy is less than ±2 dB in the attenuation of 20 dB.