Abstract: This paper presents the coaxial interlaced disc-loaded slow-wave system of the relativistic traveling wave tube(TWT). The dispersion function of the slow-wave structure is deduced by variation method and Bessel function orthogonatity without regard for thickness of disc. The paper puts much emphasis on discussing the changing of the dispersion curves following several different parameters' variety, comparing three different structures' influence upon the dispersion characteristics. The results show that this coaxial interlaced structure could make the bandwidth wider than other non-coaxial structures.