Abstract: Supercritical CO₂ extrusion foaming coupled with air-cooled die-face pelletizing technology was employed to prepare polypropylene (PP)/polystyrene (PS) composite foamed beads with three different diameters. The influence of bead diameter on the electromagnetic shielding effectiveness of isolation structures was investigated. A segregated conductive network was constructed by surface impregnation and coating with carbon nanotubes (CNT), followed by epoxy resin-assisted hot-press molding to obtain a lightweight electromagnetic shielding foam with a segregated structure. The effects of CNT content, compression ratio, and bead diameter on the electromagnetic shielding performance of the PP/PS composite foams were systematically investigated. The results indicate that increasing the diameter of the foamed beads leads to higher shielding effectiveness at lower CNT loadings. By constructing asymmetric conductive network structures, the impedance matching at the air-material interface on the electromagnetic wave incident side was significantly improved. This reduced the reflection coefficient (R) to 0.29 and increased the absorption coefficient (A) to above 0.70, successfully achieving an absorption-dominant shielding mechanism. The electrical conductivity of the uniform conductive network increased with increasing CNT content, decreasing compression ratio, and increasing bead size. This study provides valuable guidance for the design of lightweight electromagnetic shielding materials with low reflection and high absorption.