Abstract: In wide band klystrons with high perveance electron beam, the charged particles show obvious transverse movement, and the energy exchange between kinetic electrons and gap field is a nonlinear interaction process. Fully considering these physical phenomena, 2.5 dimensional large signal program was put forward the corresponding theoretical formulae based on ring model. The program was further developed for beam-wave interaction analysis. As an example, a practical S-band klystron was investigated with this tool, and the results approach the experimental data with higher precision than a one dimensional program does.