Abstract Due to the electromagnetic transparency of expanded polystyrene (EPS), the cement-based composites filled with EPS beads can be regarded as a type of “porous” material. The electromagnetic reflection loss of this porous composite material in the frequency range from 8 to 18 GHz was studied experimentally in this paper. Findings show that the filling of EPS beads can improve the reflection loss of plain cement material greatly and the EPS filling ratio, EPS bead size and sample thickness all have remarkable effects on the electromagnetic wave reflection loss of this porous composite material. With a EPS filling volume concentration of 60% and EPS size of 1 mm, the reflection loss is all higher than −8 dB in 8–18 GHz and the bandwidth for −10 dB reaches 6.2 GHz for a sample with the thickness of 20 mm. This composite material still has a relatively low bulk density and can be handled easily.