Abstract: In order to obtain the desirable output electron beam from the electron gun of transmission electron microscope, the effect of the number of plates(N) on the electron beam is studied. Through the computer modeling and the simulation analysis, the electron acceleration effect, the intensity of the electric field, and the quality of the electron beam are evaluated for different plate numbers. It is shown that the average electric field is uniform with high intensity when N=6, the electrons have the highest speed under the same acceleration voltage, and the focusing performance of the electron beam is good. The determination of the optimal number of plates can provide a reference and a basis for the specific structure design of other electron guns.