Abstract: The advances in oil-soluble viscosity-reducing agent for viscous crude oils investigated and used abroad and in China are reviewed.It is so less investigated and developed abroad and in China for oil-soluble viscosity-reducing agent for viscous crude oils that it is a new field, the prospect of which is optimistic for it is being injected into viscous crude oils directly to evasive the shortage of emulsification (such as evaporatingwater).Functioning mechanisms and structural features of oil-soluble viscosity-reducing agent, property temperature in practical use, compounding with other viscosity-reducing method are investigated in the paper.