Gyrotron Traveling Wave Tubes(Gyrotron-TWT) have both high power and broadband characteristics, which have broad application prospects in important military fields such as millimeter wave detection and imaging radar, electronic countermeasures, etc. The gyrotron traveling wave amplifier with large orbit electron beams can operate in high-order harmonic state, greatly reducing the operating magnetic field and even achieving superconducting free operation. It can improve the flexibility and maneuverability of Gyrotron-TWT. This paper optimizes and designs a Ka band second harmonic Gyrotron-TWT with large orbit electron beams, which adopts a longitudinal slot interaction highfrequency structure with dielectric loading to effectively suppress backwave oscillation and improve the stability of device operation. The process of beam wave interaction in the Gyrotron-TWT was simulated by using three-dimensional particle simulation software. The results show that at the condition of the electron beam voltage of 70 kV, the current of 6.5 A, the magnetic field can be reduced to 0.642 T, the corresponding output power can reach 106.5 kW, the bandwidth is 2.1 GHz, and the maximum gain is 35 dB.