Coupling processes of electromagnetic pulse into cavities with slot are investigated by Finite Integral(FIT) method. Slots with dimension of several millimeters are selected as research objects. The coupling functions about parallel distribution and vertical distribution of the slots are analyzed with the incident angleθ=0o. It is found that electromagnetic energy coupling into the cavity in the parallelly distributed slots would behave the second resonance effect at the point of the internal slot. At the same time, the peak electric field is only a few volts per meter at the point of the internal slot of the vertically distributed slots. Therefore, this distribution can effectively reduce the interference of electromagnetic pulses on electronic devices.