Abstract: An oil-soluble tetramer (maleic anhydride - styrene-octadecyl acrylate – acrylamide) viscosity reducer MSAZ was synthetized for Gudong heavy crude in Shengli oilfield with high amount of asphaltene and resin. Ratio of monomers, reaction temperature, reaction time and quantity of initiator were studied to find the best synthesis conditions. The best synthesis conditions are 85 ℃ and reaction time of 6h at the monomer ratio of n (octadecyl acrylate) : n (styrene) : n (maleic anhydride) : n (acrylamide) = 4.25:3:2:0.3. At 50 ℃, viscosity reduction rate could reach to 75.8% with 500ug/g MSAZ dosage. Surfactants such as lauryl sodium sulfate (SDS) were mixed with MSAZ, respectively to enhance viscosity reducing ability. At the test temperature of 50 ℃, viscosity reduction rate reaches to 85.2% using 500 ?g/g MSAZ mixed with 0.7% SDS. Surfactants with acidic functional groups in ends show a higher viscosity reduction rates than those with basic functional groups in ends. Molecular weight and distribution of MSAZ analyzed by GPC are that Mw is 20~30 thousands, Mn is 10~20 thousands and polydispersity index is about 2. The experimental results show that within the polymer intrinsic viscosity η of around 11~12 mL/g, MSAZ gives the best viscosity reduction results.