In THz range, most of the folded waveguide slow-wave structures are fabricated by microfabrication processes. The principal microfabrication processes for fold waveguide structure nowadays are discussed. The influence of microfabrication process errors including the depth a and sidewall profile on the high frequency characteristics of 0.41 THz folded waveguide slow-wave structures are analyzed. Through the analysis, the depth a shows great influence on the performance of folded waveguide Travelling Wave Tubes(TWTs), therefore, it should be controlled accurately in the process. The sidewall profile has little effect on the performance of folded waveguide TWTs when the angle is above 89°. The key processes are determined by simulation, which is helpful for developing THz folded waveguide TWTs.