The application scale of chemical viscosity reduction and cold recovery technology for heavy oil in Shengli Oilfield is increasing year by year. However，the influence of viscosity reducers on the physicochemical characteristics of heavy oil is not clear.By collecting heavy oil samples from typical blocks，the four-component analysis，relative molecular mass test，interfacial rheological property evaluation，and microscopic morphology characterization of heavy oil before and after viscosity reduction were carried out. The results show that the asphaltene content is the main factor affecting the viscosity of heavy oil. Viscosity reducer molecules weaken the interaction between resin and asphaltene aggregates through diffusion，penetration，and adsorption，resulting in a decrease in relative molecular mass. The double adsorption process of the oil-water phase under the action of viscosity reducer molecules causes a decrease in interfacial film strength and increases the deformability of oil droplets. After the viscosity reducer treatment，the cluster-like aggregate structure in the heavy oil sample is destroyed. The dispersed phase of resin and asphaltene changed to the liquid phase of saturated and aromatic phenol，and the crude oil fluidity is significantly improved.