Abstract: In this paper, the system generated electromagnetic pulse (SGEMP) phenomena caused by photoelectrons emission in cylinder cavity was simulated with FDTD and PIC methods. The characters of electric field waveforms and space distributions were analyzed and summarized in the monoenergetic photoelectron circumstance. Then, with the methods of particle sample and particle emission at a proper interval, the results were obtained when the photoelectrons emit with specific spectrum. At the same time, the results attained from two different particle simulations show that the above methods can decrease the particle numbers involved efficiently except a little bit noise increasing. Therefore, it can be used in the 3D particle simulation. The results also reveal that the length of strong electric