Abstract: To investigate the coupling effect of cavity with apertures in multiple sides against oblique incident plane wave, the equivalent transmission line method is modified based on vector analysis and field intensity distribution within a rectangular waveguide, then the shielding effectiveness of anywhere in the cavity with apertures in multiple sides is calculated. Numerical results are in agreement with the results of CST and results of experiment. The equivalent transmission line method costs less computation time and memory than CST. The method is used to evaluate shielding effectiveness of cavity with the same volume but different dimensions, the results show that: the shielding effectiveness of cavities with different dimensions are the same, but the minimal shielding-effectiveness point of cube is less than that of cavity with other dimensions, so the chance of electronic devices being interfered by electromagnetic wave is reducd.