Abstract: In order to analyze the influence of grounding locations on the shielding effectiveness (SE) of shielded room, numerical simulation is done by applying parallel time-domain finite-difference (FDTD) method on the coupling problems between shielded room and two high power electromagnetic environments,that are lightning electromagnetic pulse (LEMP) and altitude electromagnetic pulse (HEMP), respectively. Inner coupled electromagnetic (EM) fields are calculated when the shielded room is with different grounding locations. The results indicate that the farther the distance between the grounding location and the aperture, the better the SE of shielded room.