Traveling Wave Tube(TWT) bears high gain, wide bandwidth and large output power. However, the output power will decline significantly when the operation frequency rises to Terahertz range. In order to increase the output power of THz TWT by parallel multi-beam and power combination method, theoretical analysis and numerical simulation of parallel multi-beam D band Folded Waveguide-TWT(FW- TWT) are performed. The simulation results demonstrate that the method can achieve the power combining output of multi-amplified signals. According to the simulation, single-beam mode behaves flat dispersion characteristics between 0.135 THz and 0.157 THz; the 3 dB gain band is 13 GHz(0.135 THz-0.147 THz) and the largest gain is 20.88 dB at 0.14 THz; multi-beam mode gets 20.8 dB combining gain at 0.14 GHz and the combining efficiency is not below 92% in 3 dB gain frequency range. The structure is fabricated by micro milling with a dimension error below the designed values. The transmission characteristics of D band Two Beams Folded Waveguide Structure(TBFWS) are simulated and tested. The study shows important meaning for achieving high power THz radiation based on nowadays heat cathode level.