Abstract: In order to increase the output power of THz traveling-wave tube(TWT), by parallel multi-beam and power combining method, theoretical analysis and numerical simulation of parallel multi-beam D band folded TWT waveguide have been done, and simulation results demonstrate that the method can achieve the power combining output of multi-amplified signals. The results show: the single-beam mode TWT owns flat dispersion characteristics between 0.135 THz and 0.157 THz. The 3 dB gain band is 13 GHz and the largest gain is 20.88 dB at 0.14 THz. The multi-beam mode TWT got 20.8 dB combining gain at 0.14 THz and the combining efficiency is not below 92% in 3 dB gain frequency range. The parallel multi-beam mode TWT has many merits, such as high output power, small single-beam current and lower focusing magnetic field. The study helps achieving high power THz radiation basing on the present heat cathode level.