Abstract: The coupling processes of electromagnetic pulse with narrow slot and cavity with slot are simulated using the finite-difference time-domain (FDTD) method. For analyzing the variation of energy with the transmission path and time, the coupling transmission coefficient of energy is defined. When narrow slot and cavity with slot are excited by electromagnetic pulse, the resonant characteristics of coupling energy are obtained. If the width of slots is smaller than 0.2λ，it is found clearly that the energy of electromagnetic pulse coupling through narrow slots will have regular fluctuation according to the thickness of narrow slots.