Abstract: Based on electromagnetic topology and equivalent circuit method, this paper presents an analytic method for the fast prediction of the load response of the transmission line in an enclosure with apertures, irradiated by an external plane wave. In the method, the whole electromagnetic coupling problem is firstly decomposed into two relatively simple sub problems: a) the external plane wave irradiates an empty enclosure with apertures and generates interior coupling EM field, and b) the coupling field irradiates the isolated transmission line and generates the load response. Then, the coupling field is computed by equivalent circuit method, and the load response of the transmission line is computed by BLT equation. The validity of the analytic method is verified by CST simulation tool. The results show that the analytic method can calculate the load response of the transmission line fast and efficiently, and costs much fewer computing resources compared with numerical methods. In addition, the analytic method offers designers feasibility to study influence parameters on the load response of the transmission line, while it is difficult with numerical methods.