Abstract: This paper analyzes the characteristics of the front rising electromagnetic pulse and introduces the basic theory of transmission line method used to analyze shielding effectiveness of rectangular cavity. The basic formulas are then revised to calculate the case when a piece of PCB is set in the rectangular cavity. The revised formulas are expanded to arbitrary angle of polarization. Computation and simulation results indicate that: for a frequency lower than the dominant frequency, the closer the observational site is to the enclosure, the worse the shielding effectiveness is, and the shielding of low frequency is better than that of high frequency; for the same areas, the shielding effectiveness of a single hole is worse than that of an aperture array; in the same conditions, the shield