Abstract: The sheet beam klystron is a new kind of microwave vacuum device, which uses a high aspect ratio, thin rectangular beam to reduce space charge effects, increase power capability and beam-wave interaction efficiency. In this paper, an X-band SBK cavity was designed. The fundamental mode had an almost ideally flat field. 3D simulation platforms of beam-wave interaction system with plane symmetry were established to suppress the modes propagating in the drift spaces, and choke cavities and slotting the drift tube was also discussed. The results indicate that the barbell-shaped cavity could meet the need of sheet beam klystron. The cavity sizes, the height of waveguide, the width and the height of coupling cavity are sensitive to the field flatness and the drift tube noses significantly improv