An oil-soluble nanocomposite copolymer was prepared by free radical polymerization, using octadecyl acrylate, 4-vinylpyridine and acrylamide as monomers, azobisisobutyronitrile as initiator and toluene as reaction solvent, graft copolymerization with nano-silica. The nanocomposite copolymer can reduce the viscosity of heavy oil. The influences of monomer ratio, SiO2 mass ratio, viscosity reducer dosage, temperature and shear rate on viscosity reduction were investigated by single factor experiment. The performance of viscosity reducer is evaluated by the chenping heavy oil of shengli oilfield. The results show that nanocomposite copolymer is effective in reducing the viscosity of chenping heavy oil and superior to nano-SiO2 and EVA on the market. When the addition of viscosity reducer is 700 mg/kg, apparent viscosity reduction rate of heavy oil is 69.1%, and the net rate of viscosity reduction is 34.0%, under the test condition of 50 ℃, 10 s-1.