Abstract: Due to the growing demand for oil resources and the rapid exploitation and depletion of conventional light crude, low-grade heavy oil reservoirs are increasingly valued in all countries, and one of the global research focuses is how to exploit and utilize the heavy oil resources efficiently and economically. In this paper, the main components and physical properties of conventional heavy oils were introduced, the viscosity reduction methods and theories and their research progress were described, and the field application of these methods and theories was discussed. Then, based on the previous research results on mixing light viscosity reduction, the feasibility of applying vegetable oil residue and its derivative (i.e., biodiesel) to viscosity reduction of heavy oil was discussed and analyzed. It enriches the viscosity reduction method system and provides the theoretical and practical support for the research and application related with heavy oil viscosity reduction. According to the development trend of mixing light viscosity reduction technology, one important research direction is to take the biodiesel made from low cost, environmentally friendly and renewable oil foots soap foots as the diluting oil for heavy oil for future development.