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The conductive polymer composites (CPCs) with highly efficient electromagnetic interference (EMI) shielding effectiveness (SE) are always accompanied with excessive reflectivity, which would cause serious secondary EMI pollution. In this regard, the significant reduction of EMI reflection of CPCs to alleviate secondary pollution is deemed to be very important. Herein, a promising cyanate ester (CE) based composite was successfully fabricated by compounding carbonized spent coffee grounds (C-SCG) and graphene nanosheets (GNSs) via a facile solution blending followed by a hot-pressing method. Benefiting from the porous structure of C-SCG and the layered structure of GNSs, a three-dimensional (3D) multi-interface conductive network in the CE was easily constructed. The EMI SE of the resultant 9 wt% C-SCG/CE composite (C9) is 15.38 dB and dramatically enhanced to 31.09 dB with the presence of 3 wt% GNSs. The remarkable enhancement is mainly attributed to the formation of the efficient conductive pathways as well as the well-dispersion of the incorporated fillers. Meanwhile, the absorption-dominated shielding mechanism in the prepared composites gets benefit from the synergistic effect of porous C-SCG and lamellar GNSs, which effectively captures and attenuates electromagnetic waves. These encouraging findings extend the practical applications of porous biocarbon materials in EMI shielding field.