Through simulation of charcoal, the effect of remnant carbon on the rheological properties of cement paste added with FDN(polynaphthalene sulphonate) superplasticizer was studied. The effect of remnant carbon content on adsorption affinity of FDN superplasticizer for cement particles, and fluidity, Marsh time, saturation point, viscosity, and shear stress of the cement paste was investigated. The results suggest that remnant carbon really has adverse effect on the rheological properties. With increasing carbon content, flow property of cement paste shows a downtrend. With reduced fluidity, the Marsh time and saturation point increase, and the dispersibility of cement particles descends, the flocculation structure of cement paste is increased and strengthened. And the apparent adsorption amount of FDN superplasticizer on cement particles increases, shear stress and colloidal viscosity increase. The adsorption amount of FDN superplasticizer shows an inverse relation to fluidity of corresponding cement paste.