Abstract: While the operation frequency of the travelling wave tube increases, especially in the THz, the travelling wave tube faces more challenges, the attenuator is one of the most important parts. This paper designs an attenuator for the D band TWT from the choice of the materials and the optimization of the structure, with the help of CST. The cold test shows that the reflectance and the loss of the attenuator satisfies the need of the TWT. It bases the manufacture of the D band travelling wave tube.