An analytical model for double-layer shielding metallic enclosure with inner strip-shaped metallic plates is proposed for analyzing the shielding effectiveness of complex enclosures containing multiple spatial structures, based on the actual situation that the circuit systems in each spatial region are inevitably interfered by the electromagnetic penetration of those in adjacent spatial regions through slots, cables, and other transmission channels. The near-field electromagnetic interference of the external layer of the enclosure is represented by the equivalent electric dipoles, and the internal layer of it is regarded as the target point of shielding effectiveness, the analytical formulas of the internal layer's electromagnetic field then are derived based on Bethe's small aperture coupling theory and generalized Green's function. The model is employed to analyze the influences of some parameters of the strip-shaped metallic plate on the shielding effectiveness. It is shown that the position and direction of the strip-shaped metallic plate have obvious influence on the shielding effectiveness of the target point of the internal layer, embodied by the shielding values over some frequency ranges and different resonant modes, and at the same time the corresponding physical mechanisms are also given in detail. Comparison with the full wave simulation software CST has verified the model over a very broad frequency range, which provides a theoretical reference for the rapid calculation for shielding effectiveness of complex enclosures.