Abstract: In this paper, the beam dynamics in micropulse electron gun (MPG) is studied using SEEG (Secondary Electron Emission Gun) program. First, without the consideration of the space charge effect, the process of longitudinal direction beam bunching in MPG is given. Second, the formation of the saturation current affected by the space charge effect and bunching effect is studied in details. Then, the design and optimization of the micropulse gun cavity and the simulation results of the cavity by SEEG are given. At last, the scheme of a experimental micropulse gun designed by Tsinghua University is introduced.