Incorporating suitable wave-absorbing agents into cement and performing appropriate electromagnetic modification is an effective way to realize electromagnetic wave-absorbing. In this paper, cement-based composites were prepared with slag, graphite and ferrite as wave-absorbing agents, and mechanical property test, microstructure test and reflectance test were carried out to investigate the effects of different dopants and sample thickness on the wave-absorbing properties and mechanical properties of cement-based composites. The results show that the cement-based composites have the best wave-absorbing performance when the doping of slag is 30% (mass fraction) and graphite is 5% (mass fraction), and there exists the best matching layer thickness. The reflection loss is -40.84 dB at 2.65 GHz when the thickness is 36 mm. When the cement-based composite is composed of 30% slag, 10% (mass fraction) ferrite, and the matching layer thickness is 36 mm, the cement-based composites have the lowest reflectivity of -26.63 dB at 2.53 GHz. Both slag and ferrite reduce the strength of cement-based composites, and graphite (no more than 5%) is able to fill in mortar pores at the early stage of cement hydration, which enhances early mechanical properties.