Electromagnetic (EM) wave absorbing cement-based composite has promising applications in protecting civil and military buildings from electromagnetic interferences. A new idea of preparing EM wave absorbing cement-based composite is proposed by using ceramsite containing iron oxide as EM wave absorbing functional aggregate. The ceramsite was synthesized by adding 10 wt％ Fe3O4 into clay and sintering at 1200 ℃, which shows obvious dielectric and magnetic loss properties for electromagnetic wave. The maximum reflection loss (RL) of the concrete specimens prepared with the ceramsite is between -10.2--10.7 dB (corresponding to absorption greater than 90％ EM energy) in the bandwidth of 8-18 GHz. In addition, the compressive strength at 28 days age of the concrete is 46 MPa, showing the potentiality of being used as structural components in buildings.