This paper studies an electromagnetic pulse coupling to a device from a wire penetrating a cavity aperture by applying the transient electromagnetic topology (TEMT) method. The computation process is divided into two steps: The three dimensional finite-difference time-domain (FDTD) method is used to compute the excitation fields of the wire, and then the SPICE model of the wire is used to compute the current response of the device. The results obtained by the TEMT method are compared with those obtained numerically by using the FDTD method only and experimentally. And good agreement has been obtained. All the calculations are done in the time domain, and the TEMT method can be used to directly analyze nonlinear or time-varying loads. The wire does not have to be meshed in the TEMT method, so the TEMT method is more efficient than the FDTD method only.