To reduce the viscosity of highly-viscous oil of the Tahe oilfi eld (Xinjiang, China), an oilsoluble polybasic copolymer viscosity reducer for heavy oil was synthesized using the orthogonal method. The optimum reaction conditions are obtained as follows: under the protection of nitrogen, a reaction time of 9 h, monomer mole ratio of reaction materials of 3:2:2 (The monomers are 2-propenoic acid, docosyl ester, maleic anhydride and styrene, respectively), initiator amount of 0.8% (mass percent of the sum of all the monomers) and reaction temperature of 80 oC. This synthesized viscosity reducer is more effective than commercial viscosity reducers. The rate of viscosity reduction reached 95.5% at 50 oC. Infrared spectra (IR) and interfacial tensions of heavy oil with and without viscosity reducer were investigated to understand the viscosity reduction mechanism. When viscosity reducer is added, the molecules of the viscosity reducer are inserted amongst the molecules of crude oil, altering the original intermolecular structure of crude oil and weakening its ability to form hydrogen bonds with hydroxyl or carboxyl groups, so the viscosity of crude oil is reduced. Field tests of the newly developed oil-soluble viscosity reducer was carried out in the Tahe Oilfi eld, and the results showed that 44.5% less light oil was needed to dilute the heavy oil to achieve the needed viscosity.