Abstract: The influences of the dimensions of folded waveguide traveling-wave tube (TWT) on its high-frequency characteristics are investigated by the equivalent circuit method and simulation software, and the size of a Ka-band TWT is determined based on the investigation. A two-stage folded waveguide TWT is modeled by 3D particle in cell code MAG IC3D. The effect of a circuit sever on folded waveguide TWT's saturated output power and the gain growth rate of the second part circuit is simulated and analyzed. At the end of this paper, a two-stage folded waveguide TWT whose operating frequency band is in the 33 ~36GHz is designed. Simulations show that the amplifier produces a maximum radiation power of 670W at 35GHz, the corresponding electronic efficiency up to 7. 55%, and the fluctuation of the output power is less than 1dB.