Abstract: In order to study the influences of the incident electromagnetic wave and seam parameter on the rectangular cavity of shielding effectiveness (SE), SE analysis of cavity based on the transmission law combined with equivalent transmission line method was performed. The transmission into cavity of electric field was derived in detail, we took the transmission electric field as equivalent voltage source and revised the traditional transmission line model, and the calculation formula of this method was extended, it could analyze different shapes, hole array, hole spacing and loss, etc. Research shows that hole located in the center of the body wall of SE was worse than that near the body wall edge area. Compared with incident angle and azimuth angle, polarization angle has greater impact on the cavity SE. When keeping hole array area unchanged, decreasing aperture diameter, which increases the number of holes and hole spacing, can improve the cavity SE. Greater loss factor of cavity can induce better resonant frequency suppression effect. By analysis of cavity resonance frequency with theoretical and numerical methods, the result obtained by modification and expansion of the analysis method is credible.