Based on the principle of phase velocity tapering or stepping output circuit, using the phase velocity variation of the output circuit, four kinds of nonuniform output circuit models have been presented in broadband helix Traveling-wave tubes(TWTs). Taking an example of6–18GHz, 100 W broadband helix TWT and using the improved one-dimensional large signal program, second harmonic suppression performances have been studied by calculating the quantitative relationship between fundamental efficiency, second harmonic output power, and the stepping(tapering) position and strength of phase velocity variation for the four kinds of nonuniform output circuits. From the research results of backward wave and second harmonic suppression feature in two kinds of nonuniform output circuits, we obtained the design principle of phase velocity stepping or tapering, which could suppress the Backward wave oscillation(BWO) and reduce the output power of second harmonic for broadband high-power helix TWTs. Based on the principle of phase velocity tapering or stepping output circuit, using the phase velocity variation of the output circuit, four kinds of nonuniform output circuit models have been presented in broadband helix Traveling-wave tubes (TWTs). Taking an example of 6-18GHz , 100 W broadband helix TWT and using the improved one-dimensional large signal program, second harmonic suppression performances have been studied by calculating the quantitative relationship between fundamental efficiency, second harmonic output power, and the stepping (tapering) position and strength of phase velocity From the research results of backward wave and second harmonic suppression feature in two kinds of nonuniform output circuits, we obtained the design principle of phase velocity stepping or tapering, which could suppress the Backward wave oscillation ( BWO) and reduce the output power of second harmonic for broadband high-power helix TWTs.