Abstract: Properties of traveling wave-beam interaction in a thick helix slow-wave structure (SWS) are investigated for a round electron beam. The hot dispersion equation is obtained using the self-consistent field theory, and the small signal analysis is carried out including the effects of the electron beam parameters and the helixs thickness. The numerical results show that the bandwidth and the small-signal gain of the thick helix traveling wave tube (TWT) increase with the increasing beam current and beam radius; while the beam voltage does not obviously influence the small signal gain. And the small-signal gain increases as the thickness of the helix SWS structure increases, however, the bandwidth broadens a little. The presented analysis will be useful for the design of the high power TWT with thick helix circuit.