The high-frequency slow-wave structure of 2-18 GHz ultra-wideband traveling-wave tube (TWT) is studied and analyzed to meet the requirements of modern information warfare for TWT. Based on the traditional wideband TWT, the positive dispersion characteristics of non-fin loading section were introduced to realize the 2-18 GHz ultra-wideband high-frequency slow wave structure, with the maximum bandwidth of 9∶1. Results show that the output power of fundamental wave is up to 100 W, the second harmonic suppression ratio is better than -3 dBc in the full frequency band, and the second harmonic suppression ratio is better than -5 dBc at 2 GHz, which provides a theoretical basis for the design of ultra-wideband high-power devices. At the same time, the spiral pitch at the output end is adjusted to a positive gradient distribution to further optimize the low frequency secondary wave suppression ratio and improve the saturation output power of the high frequency band.