The influence of dispersion of carbon fibers in carbon-fiber-reinforced cement-based composites (CFRC) on the mechanical properties of the composites was discussed.The microstructure of the fracture surface of the CFRC samples was observed with a scanning electron microscope(SEM).The electromagnetic interference (EMI) was evaluated indirectly by reflectivity in the Naval Research Laboratory(NRL) testing system.The reflectivity of the electromagnetic radiation by the composites was measured in the frequency range of 8.0- 18.2 GHz for different carbon fiber contents of 0.2%,0.4%,0.6%,0.8%,and 1.0%(in wt pct),respectively. The results showed that the reflectivity decreased with the growing fiber content till the percentage of 0.6%. The minimum reflectivity was -23 dB,far less than -10 dB,and the composites were strong wave absorbers. After this percentage,the curve increased abruptly as the fiber content proceeded.The electromagnetic waves were gradually reflected.When the fiber content reached 1.0% finally,the maximum reflectivity -7.5 dB appeared and there was stronger reflection.The shielding mechanism includes mainly reflection,absorption, and multiple reflections. The influence of dispersion of carbon fibers in carbon-fiber-reinforced cement-based composites (CFRC) on the mechanical properties of the composites was discussed. The microstructure of the fracture surfaces of the CFRC samples was observed with a scanning electron microscope (SEM) The electromagnetic interference (EMI) was due indirectly by reflectivity in the Naval Research Laboratory (NRL) testing system. The reflectivity of the electromagnetic radiation by the composites was measured in the frequency range of 8.0-18.2 GHz for different carbon fiber contents of 0.2 %, 0.4%, 0.6%, 0.8%, and 1.0% (in wt pct), respectively. The results showed that the reflectivity decreased with the growing fiber content till the percentage of 0.6%. The minimum reflectivity was -23 dB, far After than percentage, the curve increased abruptly as the fiber content proceeded. The electromagnetic waves were gradually reflected. When the fiber content r eached 1.0% finally, the maximum reflectivity -7.5 dB had and there was strong reflection. The shielding mechanism includes mainly reflection, absorption, and multiple reflections.