Gyro-TWT is one of the most promising candidates for the transmitter microwave source of the next generation imaging radar; meanwhile, it plays an important role in national security. Gyro-TWT with helical waveguide is capable of generating broad-bandwidth radiation and highly stable. In this paper, we derive the dispersion equation of helical waveguide and the non-linear theory for calculating the beam-wave interaction. Numerical stimulations basically accord with the experimental results. We design a W-band Gyro-TWT operating with a 80 keV, 5 A electron beam, producing an output power of 142 kW with 3 dB bandwidth 4.5%, central frequency 95 GHz, and saturation gain 52 dB. In the end, we calculate the effects of the changes of voltage, current and input power on the output performance of Gyro-TWT.