This paper propose a new method for improving shielding effectiveness (SE) of a rectangular enclosure with multiple circular apertures. In this method in order to compensate the effects of the apertures on (SE) parameter, we use two metallic walls each containing apertures instead of the one wall. By using two walls for shielding an enclosure, the shielding effectiveness of the enclosure will improve. The numerical simulation uses a symmetric condensed node of TLM-TD (Transmission line Modeling Method-Time Domain) and subsequent Fourier Transform and the result was verified by HFSS and CST software. We obtain the shielding effectiveness response to an electric field impulsive excitation. It is shown that field polarization and enclosure direction has a noticeable effect on SE that should be considered in SE calculations. The effect of different parameters and improving the shielding effectiveness of a metallic enclosure with extra wall is studied. An optimization of the influence of the place of apertures in the walls and the diameter of circles on two walls is presented.