According to Equivalent Circuit Model and Electromagnetic Topology Theory, a method of evaluating the shielding effectiveness(SE) based on BLT equation is proposed, and the basic principle of BLT equation is introduced, the BLT equation of cavity-seam coupling described is derived,the function relationship between SE and related parameters such as enclosure dimensions, aperture dimensions is not only built up, but also can be used in an extension situation including off-centre aperture and arbitrary polarization direction. The effect of the aperture shape, aperture area, and the test point on SE are also analyzed. The SE of single aperture is simulated within a frequency domain from 0 GHz to 1 GHz, compared with the CST results. The results indicate that the relationship between SE and the frequency of incident wave can not only be described accurately by this method, but also can the resonant frequency of the shielding enclosure be estimated effectively.