Abstract: Aperture coupling is a main coupling mode through which ultra wide band(UWB) electromagnetic pulse(EMP) can disturb or damage many equipments. Coupling effects of slots with different shapes in a cuboid object are studied using finite difference in time domain(FDTD) method. Selecting slots with dimension of several millimeters as research object, coupling effects by varied incident angles and varied pulse widths are analyzed. Interior resonance effect, increase effect around slots and several distributing rules of electric field in the cavity are got. The main results include: clear increase effect occurs around rectangle slot; coupling coefficients of triangle slot and array are obviously smaller than that of other slots; coupling effect of incident pulse with short pulse width is obvious