Taking C.I. Disperse Blue 79 as the simulation object, the dispersion mechanism of polycarboxylic acid dispersant (poly(SMA-g-IP-MPEG)) for disperse dyes was studied by molecular dynamics simulations. The results show that the hydrophilicity of the dispersant mainly depends on the maleic acid residues; the ionized carboxylate groups of the maleic acid residues provide negative charges that make the dyes repel each other; the hydrophobicity of the styrene residues is the driving force that makes the dispersants adsorb to the dye surfaces; the branched chains grasp the dye crystals like claws, which make the dispersants combine with the dye crystals better. When the ratio of main chain monomer is 3:1 and the branched chain length is larger than 12, the branched chain tends to wrap around the main chain. The repulsion between dyes decreases, thus the dispersion performance becomes worse. When the ratio of main chain monomer is 1:1, the dispersion effect increases with the increase of branched chain length. But when the branched chain length increases to 16, the longer branched chain can adsorb with two dye crystals at the same time, which leads to the decrease of dispersion effect. In general, the main chain monomer ratio had little influence on the dispersion effect. The dispersant with the main chain monomer ratio of 1:1 and the branch chain length of 12 had the best dispersion effect on the dye. The experimental results verified the correctness of the simulation results. The design of polymeric dispersants for disperse dyes should consider not only the improvement of hydrophilicity and charge repulsion to maintain the dispersion stability of disperse dyes in water, but also the binding force between dispersants and disperse dyes. The two interactions determine the dispersion performance of the dispersant for disperse dyes.