An electron gun with multiple focus system was designed, and a smaller beam spot was obtained in the computer simulation technology. The different cathode emission models would be actived when different voltage was load on the gird electrode, and which affects the number of electrons emitted from cathode significantly, the number of electrons would increase and decrease under the Schottky emission and Rejecting-field emission mode. When voltage of focusing electrodes is U1∶U2∶U3∶U4∶U5=5∶8∶15∶70∶100, the spot size of 160 mm at the distance of 10 m was acquired in the electron gun model.