This paper focused on the preparation of the grafted modification of humic acid (HA) that was used in coal water slurry (CWS). By means of orthogonal experiment, the grafted humic acid was synthesized with acrylic acid (AA) copolymerized with humic acid, using potassium persulfate as the initiator in the reaction system. The structure of copolymer was characterized by ATR-FTIR. And then, the viscosity, liquidity, and stability of CWS used the dispersant based on the modified humic acid were determined through the CWS viscometer and tests of stability. The optimum conditions of synthesis are that the mass ratio of AA and HA is 0.35:1, dosage of initiator is 5%, reaction temperature of is 75°C, reaction time is 2.0h. Besides, when the dosage of active ingredient of the dispersant for CWS was 0.12%, the viscosity of CWS achieved lower.