Abstract: The finite-difference time-domain method is used to study the coupling of the electromagnetic pulse to the computer case through slots or holes on it.EMP with different incident directions and different polarization direction coupled into computer case is discussed respectively.Some conclusions can be obtained from simulation:(1) The energy coupled into the computer case mainly comes from the slots on cabinet's front panel;(2) The EM wave can more easily transmit down inside the computer case and energy is much more in the middle of it when EMP is oblique incidence down from above of the computer case's front panel compared with perpendicular incidence to the front panel;(3) The coupled energy when EMP's polarization direction is perpendicular to the slots is much more than that when it is parallel to the slots.