In order to solve the problem of high viscosity and poor fluidity of heavy oil in Bohai A27-2 platform during long-distance pipeline transportation，a terpolymer viscosity reducer with octadecyl methacrylate （A），styrene （S） and 2-acrylamido-2-methylpropanesulfonic acid（Xm）as monomers was synthesized. The effect of various factors on polymerization reaction was studied by measuring the oil viscosity reduction rate of viscosity reducer. The optimal reaction condition was determined and the structure of reaction product was characterized by infrared spectrometer. The experimental results showed that the optimum polymerization condition was obtained as follows：9∶5∶1.5 mole ratio of monomer A∶S∶Xm，4 h reaction time，80℃ reaction temperature，2.0% and 320% the addition of initiator and mixed solvent of the total mass of monomer respectively，and 8∶ 3 mass ratio of toluene to Ys in mixed solvent. The oil-soluble viscosity reducer had a good viscosity-reducing effect on heavy oil. The viscosity reduction rate was 59.25% when the dosage of viscosity reducer was 1‰ of heavy oil. The viscosity reduction rate of heavy oil was 82.18% after 10% composite oil-soluble viscosity reducer contained viscosity reducer and surfactant OP-10 was added，which indicated that the heavy oil could be transported through pipe at lower temperature.