This paper centers on the technical problem of aperture coupling of multi-layer shielding cavity, and carries out the research on the influence of aperture coupling on the shielding effectiveness to provide technical support for the electromagnetic compatibility design of large radio telescopes. Based on Robinson model and electromagnetic topology theory, this paper establishes the equivalent circuit of a double-layer perforated shielding cavity and its signal flow diagram, and solves the shielding effectiveness using the BLT (Baum-Liu-Tesch) equation, taking into account the case of a 3-layer cavity. The accuracy of the measurement results, simulation values, Robinson's algorithm and this paper's algorithm are compared and analyzed, and the accuracy of this paper's algorithm is verified. On this basis, this method is used to analyze the influence of the layer spacing, the shape of the hole, and the installation position of the radiation source on the shielding effectiveness of the double-layer metal cavity in the 0--1.5GHz band, and the engineering suggestions are given. In addition, the relationship between the number of layers of the three-layer shielding cavity and the shielding effectiveness is analyzed. The results show that there is a linear relationship between the shielding effectiveness and the number of layers.