Abstract: Supercritical CO₂ extrusion foaming coupled with air cooling die face pelletizing technology was employed to prepare polypropylene/polystyrene (PP/PS) composite beads foams with three different diameters. The influence of bead diameters on the electromagnetic shielding effectiveness of isolation structures was investigated. An isolated conductive network was constructed by surface impregnation and coating with carbon nanotubes (CNS), followed by epoxy resin-assisted hot-press molding to obtain a lightweight electromagnetic shielding foam with an isolated structure. The effects of CNS content, foam 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 facilitates higher shielding effectiveness at lower CNS 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 0.71, successfully achieving an absorption-dominant shielding mechanism. The electrical conductivity of the uniform conductive network increased with higher CNS content, lower compression ratio, and larger bead size. This study provides valuable guidance for the design of lightweight electromagnetic shielding materials with low reflection and high absorption.