In this paper, a 2.5-dimensional macro-particle model is proposed for a nonaxisymmetrical and large width-height ratio sheet beam klystron (SBK) and a corresponding beam-wave interaction program is developed. An X-band 100-MW SBK is simulated for testing the computing capability of the program. The results are in good agreement with those from 3D PIC software, and the computation time can greatly be reduced, which is useful for the initial design and the optimization of the SBK beam-wave interaction structure.