Abstract: In this paper, the finite-difference time-domain(FDTD) method was employed to simulate the linear coupling processes of the microwave pulse into the nested cylindrical cavities system with narrow rectangular slots. The influence of the position of the inner slot on the coupling characteristics of outer cavity and inner cavity was analyzed. Except the region near the inner slot, the influence on the coupling characteristics of outer cavity is very small and it can be neglected. However, the influence on the coupling characteristics of inner cavity is significant. The shielding property of the inner cavity is determined jointly by the distribution of coupling field inside the outer cavity and the position of the inner slot.