The invention provides a starch-based graft copolymer coal water slurry dispersant and a preparation method thereof. The method comprises steps that: a 10% to 20% starch aqueous suspension is heated; an oxidizing agent which takes 0.1% to 5% of the mass of starch is added to the aqueous suspension of starch for carrying out an oxidization reaction for 10 to 60min; the mixture is heated to a temperature of 80 to 95 DEG C, and the mixture is gelatinized for 20 to 60min while maintaining the temperature; the temperature of the mixture is reduced to 40 to 70 DEG C, and meanwhile mixed monomer and an initiator are dropped into the mixture; the mixture is heated to a temperature of 80 to 95 DEG C, and is subject to a reaction while maintaining the temperature for 20 to 120min; the temperature is reduced to 20 to 40 DEG C, and the pH value of the mixture is regulated to 7-9 by using a sodium hydroxide solution. The dispersant is a viscous emulsion with a color of light yellow to off-white, a solid content of 25 to 35%, a pH value of 7 to 9, and a viscosity of 30 to 150mPa.s. According to the invention, strong dispersibility of styrene and carboxylic salt is fully utilized. Hydrogen bonding effect of polyhydroxy on starch straight chains upon hydrones is adopted, such that the storage stability of the coal water slurry is improved, the dispersibility of the coal water slurry is good, and the stationary phase of the coal water slurry is long-lasting.