the best synthetic method of the copolymer of maleic anhydride, styrene and octadecyl acrylate(MSA), an oil-soluble heavy oil viscosity depressant, was investigated and the effects of the dosage of MSA, sodium dodecane sulphonate as adjuvant and water on the viscosity reduction of the Shengli heavy simulated oil was probed. The results showed that the best synthetic conditions of MSA were as follows, the mass ratio of solvent toluene and total monomers were 1:1, the mol ratio of octadecyl acrylate, styrene and maleic anhydride were 6:1:2, the synthetic temperature were 90℃ and the synthetic time were 4 h. When the dosage of MSA was 1.0% and the temperature was 90℃, the effect of viscosity reduction of Shengli heavy simulated oil was the best. Besides, the adjuvant of sodium dodecane sulphonate could improve the effect of viscosity reduction of Shengli heavy simulated oil evidently, and the best dosage of sodium dodecane sulphonate was 0.8%. As the content of water in raw oil increased, the effect of viscosity reduction of Shengli heavy simulated oil increased, and when the content of water in raw oil was up to 40%, the ratio of viscosity reduction of Shengli heavy simulated oil exceeded 90%.