Among various heavy oil recovery technologies, physical viscosity reduction has the advantages of simple process and good adaptability, but it has the disadvantages of high production cost and shortage of thin crude oil resources. Chemical viscosity reduction has the advantages of fast effect and low energy consumption, but it has the disadvantages of complex treatment process. Oil-soluble viscosity reducer technology combines the advantages of physical viscosity reduction and chemical viscosity reduction. It has many advantages such as less addition, good effect, low cost and simple post-treatment. In recent years, it has attracted the attention of experts in the industry. This paper introduces the reasons for the high viscosity of heavy crude oil, analyzes the viscosity reduction mechanism of oil-soluble viscosity reducer, summarizes the synthesis process of various oil-soluble viscosity reducers, and reviews the advantages and disadvantages of different types of viscosity reducer. Compared with binary and ternary oil-soluble viscosity reducers, a variety of polar groups of quaternary oil-soluble viscosity reducer can act better on macromolecules in heavy crude oil and destroy the layered structure of resin and asphaltene, so that the viscosity of heavy oil can be significantly reduced. The composite and compound use of viscosity reducer can enhance the viscosity reduction effect. The development trend of oil-soluble viscosity reducer is prospected. It is believed that it is necessary to further study the viscosity reduction mechanism at the molecular level and optimize the synthesis process of viscosity reducer from the perspective of green environmental protection. The solution of the above problems will contribute to the design, preparation and wide application of high-efficiency oil-soluble viscosity reducer molecules.