Taking the shielding enclosure of airborne electronic equipment with apertures and slots in the center as the research object,a BLT (Baum-Liu-Tesche) equation which can calculate the shielding efficiency of the enclosure under plane wave irradiation is established.Aiming at the problem of large error in Robinson aperture array impedance calculation,the eccentricity coefficient Cmk is introduced to modify the aperture array impedance,and the modified BLT equation can accurately calculate the shielding efficiency of the enclosure with aperture array at any position.The calculation of shielding effectiveness of shielding enclosure at any polarization angle is realized by vector decomposition of electromagnetic wave electric field.The results show that the shielding effectiveness of the shielding enclosure increases with the increase of the number of apertures and slots.The increase of polarization angle enhance the electromagnetic shielding ability of shielding enclosure.By comparing the calculation results of BLT equation with the results of Robinson method and CST simulation,it is proved that BLT equation is accurate,which provides an effective method for the shielding effectiveness of shielding enclosure of computer-borne electronic equipment.