Problems in the electro-optical system in high peak power multi-beam klystrons are analyzed. The electro-optical system of an S-band 50 MW klystron is designed by using an indirect method. Respective shield barrels are used to generate side-symmetrical magnetic field in the cathode-anode area. With the combination of 2D electron-optical code EGUN, 3D electromagnetic code MAFIA and 3D PIC (particle-in-cell) code, the electro-optical system is modeled in detail. The result shows that the shield barrels could generate side-symmetrical magnetic field, and that the track of electrons is perfect.