Attenuation has significant influence on folded waveguide slow-wave circuit in the terahertz range. A theoretical model calculating the loss of folded waveguide slow-wave circuit is presented, and the attenuation coefficient of the bend waveguide is derived in this paper. The loss of 0.67 THz folded waveguide slow-wave circuit is evaluated by the proposed theoretical model and commercial simulation software respectively, the results of the two methods show perfectly accordant, which indicates the theoretical model has great precision. In the final, the influence of the variation of structure parameters on the loss of 0.67 THz folded waveguide slow-wave circuit is studied by the theoretical model.