The finite-difference time-domain method is used to research the rule of the transient electromagnetic field coupled to the cavity that has many apetures on it. The coupling effects of different bandwidth and different polarized direction are described in this paper. When the polarized direction of the incident wave are parallel to the short side of the apeture, the coupled energy will attain the maximum. If the bandwith and the risetime are shorter, then the energy coupled to the cavity are greater.