The folded waveguide Traveling-Wave Tube(TWT) was served as pre-driving source of high-power gyrotron TWT in this paper. The dispersion curve of Slow Wave Structure(SWS) and coupling impedance were analyzed using EM simulation software HFSS and particle simulation software CST particle studio, and the beam-wave reaction process was simulated to obtain working parameters of gain, working voltage and current. Under the beam voltage of 13.9 kV, current of 16 mA and input power of 5 mW, the 0.14 THz wave was generated with linear gain of 30 dB, bandwidth of 3.7 GHz, maximum power of 6.2 W. The results will provide enough feeding signal power for 0.14 THz high-power gyrotron TWT to realize kW-level power.