Abstract: Finite-difference time-domain method (FDTD) was used to simulate the coupling processes of ultra-wide-band (UWB) electromagnetic pulses into a cavity with a slot, and an anisotropic perfectly matched layer (PML) absorbing medium was inserted to truncate the computational lattices. According to the symmetry of the object and examining the special nodes of grids, the effectiveness of UWB pulses coupling into the whole cavity with a slot was obtained. The results show that the uniaxial PML is a suitable absorbing boundary condition for computing the problems of pulses coupling into slots.