The loss of the folded waveguide slow wave structure is significant in the terahertz regime, which is necessary to be well studied as designing the slow wave structure of a folded waveguide Traveling Wave Tube(TWT) at 220 GHz. In this paper, the S parameters of the slow wave structure are predicted firstly by simulation, and then the sample of the slow wave structure is fabricated by Ultra Violet-Lithographie, Galanoformung, Abformung(UV-LIGA) process and measured. The measurement result shows that the attenuation factor of the sample is about 240 dB/m at 220 GHz, which is reasonably consistent with the simulation result. The micrographs show that there is a discrepancy between two results at higher frequencies due to the sample deformation.