Abstract: Aiming at the problem of complex electromagnetic interference in the work of shielded enclosures with holes. Based on the electromagnetic topology theory and the BLT equation, an analytical model for calculating the shielding effectiveness of an enclosure with oblique openings is proposed, considering the situation of opening inclination, eccentricity, arbitrary plane wave irradiation, and arbitrary observation point position. At the same time, the material of the enclosure is considered as a lossy conductor, and the scattering and propagation matrices in the BLT equation are modified. The distribution relationship of voltage and electric filed is exploited to account for the various propagation modes in the enclosure in order to extend the computable frequency band of the model to 0-3 GHz. Finally, five sets of experiments are designed to verify the accuracy and optimization of the proposed model by comparing the CST simulation values, the Robinson extended algorithm calculated values and the calculated values of the proposed model. It provides a method for prediction and calculation of shielding problems of shielded enclosures with holes in practical engineering.