Abstract: In order to study the mechanism of viscosity reduction by blending non-petroleum solvents into the residual oil and its viscosity-temperature model, the viscosity-temperature data of Liaohe vacuum residual oil was measured after shale oil, coal light oil and coal diesel were blended respectively for viscosity reduction. The experimental results show that these three light oils can reduce the viscosity of residual oil significantly. It is indicated that the association force between the basic schistose layered structures of asphaltene is the important factor influencing the viscosity reduction of residual oil by blending light oil. The theoretical equation of liquid viscosity derived by Zhang Kewu is extended to describe the viscosity-temperature relationship of residual oil blended with light oil. Based on the experiments and analysis, two mechanisms for viscosity reduction of residual oil by blending light oil were put forward, i.e., asphaltene dilution and disassociation. And a model on viscosity reduction of residual oil by blending light oil was developed, and its calculation results are in good agreement with the measured values. The model parameters reflect the composition of blending oil and the effect of light oil on asphaltene association of residual oil. And in a certain sense, this model is physically significant.