A new maleate was synthesized by polypropylene glycol and maleic anhydride under the condition that using p-toluene sulfonic acid as catalyzer . And then an oil-soluble terpolymer was synthesized by vinyl acetate, acrylamide and the maleate which was prepared before. The chemical structure of the copolymer was identified by IR spectrometric data. The optimum reaction conditions were obtained as follows : the monomer molar ratio of vinyl acetate, acrylamide and maleate was 6:2:1, with initiator dosage of 0.9%, polymerization temperature of 75℃and reaction time of 6 hours. The viscosity was decreased from 3790mPa•s to 1110mPa•s when the dosage of viscosity reducer was 600mg/L at 50℃.