A modified folded groove waveguide called folded double ridge groove waveguide(FDRGW)is put forward for developing broadband high-power terahertz (THz) travelling-wave tube (TWT). A new transmission waveguide which is appropriate for this new kind of SWS as input and output energy coupler is proposed. It can be found from the high frequency characteristic simulation results that the folded double ridge waveguide SWS can increase the average interaction impedance and extend its operating bandwidth. In addition, the particle-in-cell (PIC) simulation results reveal that with the beam voltage of 27.4 kV and the beam current of 0.25 A, the average output power of the new folded double ridge groove waveguide TWT can reach 65.8 W and the corresponding gain is 2721 dB at the center frequency 340 GHz.Therefore, the folded double ridge groove waveguide TWT could be used as wide-band and high-power terahertz radiation source.