0.14 THz Folded Waveguide Traveling Wave Tube(FWTWT) is a kind of devices with wide frequency band and high gain. But it tends to produce self-oscillation which will prevent the tube from normal working. Adding an attenuator into the tube is the core technology of suppressing self-oscillation. Considering the choice of the loss materials and the match of the structure, a kind of attenuator for the 0.14 THz FWTWT is optimized and designed with 3D simulation software by calculating the absorbability and reflection of the attenuator. The cold test shows that the match characteristic and the absorbability meet the requirements of the attenuator. It lays a foundation for the manufacture of 0.14 THz FWTWT.