Gyro-TWT is one of the most promising candidates for the application of 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, as well as high stable ability. In this paper, we derive the dispersion equation of helical waveguide and the non-linear theory of calculating the beam-wave interaction. Numerical stimulation results accord with the experimental results. We design a W-band gyro-TWT operating at a 80 keV, 5 A electron beam, which can produce an output power of 142 kW, with 3 dB bandwidth 4.5%, cantral frequency 95 GHz and saturation gain 52 dB. Finally, we calculate the effect of variation of voltage and current on the output performance of gyro-TWT with helical waveguide.