This invention discloses a method for synthesizing a high-efficiency crude oil demulsifier from cornstarch and conventional polyether demulsifier by graft copolymerization. The method comprises: (1) adding solvent and cornstarch into a reaction kettle with a stirrer, slowly adding proton acid (20-80 wt.% of cornstarch), and stirring for 1-4 h; (2) adding polyether demulsifier and solvent, and refluxing under heating for 2-8 h; (3) adding NaOH solution for neutralization, ending NaOH addition when the pH value is 6.5-8.0, and reacting for 1-3 h until the solution is transparent. The demulsifier has such advantages as simple process, mild operation conditions, wide applications, and good demulsification effect.