A novel slow-wave structure (SWS)， the folded double-ridged waveguide structure， is presented and its properties used for wide-band traveling-wave tube (TWT) are investigated. A Ka-band folded double-ridged waveguide TWT is modeled by Magic 3D and the beam-wave interaction is simulated and analyzed. Within the same beam condition， it is concluded from the comparison of folded double-ridged waveguide TWT and folded waveguide TWT that: in the condition of similar gain maximum， the folded double-ridged waveguide TWT has wider bandwidth of which is 22%， and higher electron efficiency of which is 9％.