he RMG380 marine fuel oil was prepared by vacuum residue, coking wax oil and ethylene tar, the oil-soluble viscosity depressant was synthesized by maleic anhydride, styrene and octadecyl acrylate(MSA). The optimized synthesis conditions are as follows: octadecyl acrylate, styrene and maleic anhydride monomer ratio 5∶1∶3, dosage of initiator 0.6%, synthetic temperature 90 ℃, reaction time 6 h. This synthesized viscosity depressant was evaluated under the concentration of 300 mg/L and shear rate of 2 000 r/min at 50 ℃, the ratio of viscosity reduction of marine fuel oil can reach 66.11%, and meet the ISO8217—2005 RMG380 viscosity requirements of marine fuel oil. Meanwhile, this viscosity depressant was suitable for different marine fuel oil, while viscosity reducing effect gradually weakened with the increase of time.