Carbon-fiber-reinforced cement-based composites were prepared and their fracture surfaces were observed by scanning electron microscopy. The reflectivities of the composites in the frequency ranges of 4-8 GHz and 8-18 GHz were measured by using the Naval Research Laboratory testing system to assess the electromagnetic interference through investigating the influence of carbon fiber contents of 0.2, 0.4, 0.6, 0.8, 1.0 wt% on their reflectivities. Results showed that all of the measured reflectivities data were less than -10 dB in the lower frequency range and there was mainly absorption of electromagnetic waves. However, the reflectivity was more than -10 dB when the fiber mass fraction was over 0.6% in the higher frequency range, and the electromagnetic waves were mainly reflected. As the mass fraction increased, the reflectivity changed. The minimum reflectivity in the lower frequency range was -15.1 dB when the mass fraction was 0.6%, whereas that in the higher frequency range was -19.3 dB when the mass fraction was 0.4%.