Abstract: This paper presents an electromagnetic topology model for shielding effectiveness estimation, considering oblique incidence and polarization, based on Robinson equivalent circuit theory. In this model, the apertures are represented by a two-port network, which connects the free space (represented by a transmission line with characteristic impedance Z0) and the enclosure (represented by a short circuited length of rectangular waveguide). The aperture scattering matrix is deduced by equivalent circuit model, based on which a new generalized BLT equation for aperture coupling is proposed. Four test models are designed for the comparison of the EMT algorithm, Robinson algorithm and CST. The simulation result shows that, the EMT algorithm is more accurate than the Robinson algorithm, especially in the high frequency domain; while it is as concise as the Robinson algorithm, and establishes the relationship between shielding effectiveness and parameters of enclosure and apertures, which makes it more practical than CST.