Abstract: The heavy oil of Shengli oilfield has a high viscosity and is difficult to transport. A new type of oil-soluble viscosity reducer RPJN-SL is developed, which has a viscosity reduction rate (VRR) of more than 55% for Shengli heavy oil. The mechanism of the oil-soluble viscosity reducer （VR）is studied by dynamic light scattering and X-ray diffraction. The results show that the VR molecules interact with the aggregate micelle of asphaltene and resin to reduce the micelle size and the internal force between micelles so as to depress the viscosity of the heavy oil and reduce the order of the asphalt aggregate structure as well. The influencing factors studied show that the VRR rises with increasing the concentration of VR, and is not affected by temperature and shearing. The oil-soluble viscosity reducer RPJN-SL can fulfill the demands of pipeline transportation.