A fast design method of slow-wave structure for 0.22 THz broadband folded waveguide is developed through theoretical analysis of folded waveguide, and the influences of structure parameters on the high-frequency characteristics is analyzed. In order to prevent oscillation in simulations，a truncated slow-wave structure is adopted. Interaction simulations show that the nonlinear gain is 23.9 dB and the output power is 1.2 W at 0.22 THz where the beam voltage and current are 16 kV and 10 mA, respectively. The 3 dB bandwidth is about 14 GHz(0.214 THz-0.228 THz), and the intraband output power is higher than 0.5 W, while it is higher than 1W from 0.217 THz to 0.224 THz.