Abstract: To investigate the shielding effectiveness of rectangular chamber with apertures, a vector decomposition/ equivalent transmission line model is presented, the formula of shielding effectiveness at the center point of rectangular chamber with single aperture in two sides is obtained. Numerical results are in agreement with the results of CST, and the calculation and convergence speed are much faster. The method is generalized to evaluate shielding effectiveness of chamber with multi-holes in two sides. The coupling mechanism and multi-mode resonance law are analysed. The coupling electric field of center point is enhanced, under the main coupling frequency, the shielding effectiveness is smaller with the frequency accretion; with the frequency below 2GHz,the each direction shielding effectiveness of chamber has multi-mode resonance phenomenon, and the resonance mode is different with each other;the shielding effectiveness of chamber with multi-holes is worse than single hole, and resonance mode has no obvious change.