Abstract: The folded waveguide structure which has enormous advantages can been applied into THz TWT. In the article, a simple theory and equivalent circuit theory were used to analyze high frequency performance of 0.85 THz FW -TWT, and the CST MWS and HFSS codes validate these equivalent theories. The results of simulation and theoretical calculation were coincident. Because FW is a special slow-wave structure, the highest DC voltage was got. In deviation of fabrication dimensions, optimization structure parameters which make 0.85 THz FW-TWT to have wide band and high impedance were got. The results show the limited DC voltage is 26 kV and r0/a(r0represents beam hole radius and a is the width of FW) is about 0.1. The largest cold band width is about 400 GHz and the band width in flat dispersion region is about 275 GHz (825-1 100 GHz) and the change between the lowest and the highest phase velocity is below 1%. The design approach can been used to guide the design of slow-wave structure of THz FW-TWT.