RF structure of W-band sheet beam EIK has been designed in this paper. The interaction system consists of two five-gap cavities, one is used for input cavity, the other is output cavity. The whole beam wave interaction system has been calculated and simulated by two kinds of 3D-PIC software, and the results agree well. Beam voltage of the SBEIK is 75kV, and beam current is 4A. The gain of whole tube is higher than 24dB, which is only consisted of two resonant cavities. If there are more cavities, the tube will get higher gain. This is a big progress and is very useful for the design of multi -cavities SBEIK.