Abstract: With the help of 3D PIC software, the beam-wave interaction for a 0.4 THz confocal gyro-traveling-wave tube was simulated. With the consideration of cold dispersion characteristics and diffraction loss ofthe confocal waveguide, HE06 mode is chosen as the interaction working mode. For the purpose of suppressing competing mode, a sever structure is used. In the simulation process, with the adjusting of beam voltage, beam current, working magnetic field, and velocity ratio, the optimized working parameters are found. At last, the operating parameters are chosen as 34 kV beam voltage, 14.25 T magnetic field, 2 A beam current, 0.75 velocity ratio, and with the input power of 1 W, 2.76 kW output power is achieved. The gain exceeds 34 dB, the 3 dB bandwidth achieves 8 GHz, and the efficiency achieved 4%.