Three-dimensional electromagnetic particle-in-cell （PIC） simulation is based on FDTD （finite-differential time-domain） and PIC methods. Based on the characteristics of these methods, the parallel algorithm was designed, which divided the whole simulation domain into a number of subdomains, and every computing process simulated a subdomain and transferred the boundary information. Then the influencing factors of speedup were analyzed. Finally, the parallel algorithm was implemented in CHIPIC3D software and the parallel version of CHIPIC3D was used to simulate a kind of magnetically insulated transmission line oscillator and the relative klystron amplifier. The results show that the parallel algorithm is correct and the speedup is increased.