The electromagnetic waves radiating inside a building can cause reverberation effect, which can be evaluated using power balance method (PWB) to quickly determine the field level of indoor electromagnetic environment. However, the current calculation models of wall coupling cross section (CCS) in PWB method for electricallally large enclosure are based on the assumption that electromagnetic waves cannot penetrate through the enclosure walls. As a result, these models are not applicable for calculating the CCS of penetrable indoor building walls. To address this issue, a novel CCS model applicable for building walls with finite thickness is presented. The proposed CCS model considers the thickness and electromagnetic characteristics of building walls and can effectively reflect the effects of electromagnetic wave’s multiple reflections inside the walls on the indoor electromagnetic environment. The proposed model has been employed to estimate the indoor electric field level. The predicted results agree with the measurements, which validates the proposed CCS model for building walls with finite thickness.