Abstract: In this paper the advances in oil-soluble viscosity-reducers for viscous crude oil at home and abroad are reviewed. The article analyzes the mechanisms of the viscosity forming and the viscosity reduction by oil-soluble viscosity-reducers. The viscosity-reducers are classified into three types,the synthesis methods and the application in the oil-field are presented. The article discusses the possible problems in the research and development of high efficiency viscosity-reducers, also in the applications. It indicates that introducing polar groups or active surface groups to the polymer and mixing oil-soluble viscosity-reducers with oil-soluble surface active agents or fluorinated surface active agents are the developing direction.