Abstract: This article firstly introduces the basic theory of transmission line method(TLM) which is used to analyze shielding effectiveness of rectangular enclosure with apertures,then expands the fundamental formulas to deal with cases of round hole,multiholes, and arbitrary angle of polarization. Simulation results indicate: (1)when the frequency is under dominant resonant frequency, the closer to the enclosure, the more coupling energy; (2)under resonant frequency, resonance appears between the aperture and the enclosure, which results in low even negative shielding effectiveness in the cavity; (3)sielding effectiveness decreases when polarization angle increases, and shielding of low frequency is better than that of high frequency; (4)fr the same areas, shielding effectiveness of singlehole