Abstract: Gyrotron traveling wave tubes (Gyrotron-TWT) have both characteristics of high power and broadband, which have broad application prospects in many important military fields such as millimeter wave detection and high resolution imaging radar, electronic countermeasures, etc. The Gyrotron-TWT with large orbit electron beams can operate in high-order harmonic state, exponentially reducing the operating magnetic field and even achieving superconducting free operation, thereby advancing Gyrotron-TWT miniaturization. It can improve the flexibility and maneuverability of Gyrotron-TWT. A Ka band second harmonic Gyrotron-TWT with large orbit electron beams is theoretical designed, which adopts helically corrugated waveguide loaded with lossy dielectric 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 75 kV, the current of 8 A, the magnetic field of 0.593 T, the corresponding output power can reach 140.54 kW, the bandwidth is 3.2 GHz, and the maximum gain is 30.65 dB.