Abstract: Graphene-nanosheet/polyethylene(GNS/LDPE) composite films with considerable orientation fabricated in sandwich structure were developed to investigate the electromagnetic interference shielding (EMIS) performance. The experimental results demonstrate that electrical property primarily determines EMIS function. The according theoretical calculations are performed based on well-materialized conductivity as for multilayer composite films to explored the essential mechanism of absorption and multiple-reflection for polymer-based graphene nanocomposite films. The improved EMIS performance could be obtained though modifying thickness, skin depth and electrical conductivity of active material in sandwich structure. The total shielding performance can be further improved from absorption shielding enhancement by enlarging shielding thickness. The optimized shielding efficiency achieves up to 31 dB, demonstrating significant shielding effectiveness of GNS/LDPE composite films. The shielding mechanism of nanocomposite sandwich structure presents fundamental regulation of designing high EMIS nanocomposites. The results reasonably suggest a effective technical approach to fulfill promising graphene/polymer nanocomposite films potentially in EMIS coating applications.