Abstract: This paper introduced the particular advantages of electrostatic focusing style in millimeter traveling wave tube and analyzed the methods to establish periodic electrostatic field. Based on a Ka band traveling wave tube, first we designed an intense current density electron gun and selected a comb-like shape as slow wave circuit. Then a numerical simulation on the electron optical system made up of an electron gun and a slow wave circuit was carried out. Finally we summarized tolerance for key sizes of the slow wave circuit. The results showed that one hundred-percent beam passed slow wave structure smoothly. The good static beam pass was a foundation stone for subsequent sufficient beam-wave interaction and it was proven that a large current density beam in millimeter traveling wave tube could be focused commendably by electrostatic field. Simultaneously, the summary for the tolerance could provide a reference for the design of the electron optical system of millimeter electrostatically focused traveling wave tube.