Effects of grafting densities of comb-like copolymer on the dispersion properties of concentrated cement suspensions are investigated systemicly in this paper. For comb-like copolymers the adsorbed amount increases at the same dosages as the grafting density decreases, while the efficiency of dispersion is improved by increasing the grafting densities. Based on the theory of Flatt at level of scaling law, the molecule size and the occupied surface area of the comb-like polymers with different grafting densities are calculated. The comb-like polymers with higher grafting density has bigger molecular size. For comb-like polymer dispersant, the steric repulsion provided by the side chains is the main drive of the dispersion, so the Flory energy is employed to explore the relationship between steric repulsion and grafting density. The calculation tells us that higher grafting density results in higher Flory energy which represents stronger steric repulsion. The calculation method based on the scaling law and Flory theory is successfully used to interpret the dispersion mechanism, and it is anticipated that this method can be applied to explain the effect of other parameters of PCE comb-like polymers on the dispersion properties of cement suspensions, ceramics suspensions, slurries and so on.