Abstract: After analyzing the design process with previous electron beam optics simulation and modeling tools and their shortcomings, this paper presents that MAFIA PIC simulation is a more accurate method as it can wholly support spacechargelimited (Child's law) and temperaturelimited emission, cathode thermal initial velocity and nonlaminar flow. The 3dimensional timedependent PIC code is also desired for studying the beam transient and noise character. Using MAFIA PIC code to simulate an electron gun, some useful data are obtained, which show that the simulation results are close to the experimental data within acceptable error range. It can be used to instruct the engineering design of microwave tube electron optics.