When an electronic device featuring an enclosure and an internal PCB is located inside a building, analyzing the coupling phenomenon resulting from external high-power electromagnetic waves at specific locations on the PCB using conventional commercial simulation tools requires significant computational resources. As the analysis frequency increases beyond several GHz, the number of mesh elements grows substantially, making full electromagnetic simulation practically infeasible. This paper proposes a spatially partitioned frequency-domain analysis method based on the Huygens’ box-based electromagnetic topology approach to overcome the limitations of conducting coupling analysis for small enclosures inside buildings. First, a Huygens’ box is assumed to contain the enclosure of the electronic equipment, and the coupling characteristics of the external electromagnetic waves incident through a window into the Huygens’ box are analyzed. Then, a separate Huygens’ box and enclosure are constructed to analyze the coupling effects at specific locations on the internal PCB. Additionally, a two-stage Huygens’ box is introduced to contain the PCB inside the enclosure, and the results are compared. Finally, the feasibility and validity of the proposed partitioned-recombination analysis method are verified by comparing its results with those of full-model simulations up to 10 GHz.