Abstract: This paper studied the dispersion of the gyro-TWT with a helical waveguide, and the dispersion equations were obtained by theoretic analysis. The dispersion curves were studied through numerical computation. The paper mainly analyzed how the change of the structure parameters would affect the dispersion characteristics. Reasonable structure parameters were given. By using the electromagnetic simulation software CST, the operating eigenwave of helical waveguide with given parameters was calculated and compared with theoretical data.