An improved model based on the electromagnetic topology (EMT) theory and Baum-Liu-Tesche (BLT) equation is presented to evaluate the shielding performance of an apertured enclosure. First, the total aperture impedance of arbitrarily positioned apertures can be easily obtained through the coupling factor. Then, the expression of the electric field components at any monitor point inside the enclosure is derived according to the relation between voltage response and field distribution, hence, the shielding effectiveness (SE) and resonances can be analyzed conveniently. Finally, the improved model is validated by the CST, showing high efficiency and accuracy for the prediction of the SE and resonant modes of an enclosure. The improved model is suitable for handling slots that have a large length-width ratio, irregularly arranged aperture arrays, multiple apertures with different shapes and dimensions, and cascaded triple enclosures with off-centered apertures, showing the wide application range of this model.