In order to further explore the mechanism of “increasing oil and decreasing viscosity” of heavy oil emulsification and viscosity reducer, experimental studies on oil displacement of heavy oil emulsification and viscosity reducer and its influencing factors have been carried out. Viscosity reducer was screened and related performance was evaluated.The viscosity and recovery efficiency were used as evaluation indexaccording to the geological characteristics and fluid properties in Bohai reservoir. The results showed that, the mechanism of decreasing viscosity of three kinds of viscosity reducer was forming oil in water emulsion, and the viscosity reduction effect of “viscosity reducer 2” was best among three kinds of viscosity reducer. Oil water emulsion gradually changed from oil in water (O/W) into water in oil(W/O) type and the viscosity was increased when the water content in emulsion decreased, eventually exceeding that of crude oil. Besides, with the heavy oil reservoir hetergeneity increasing, oil displacement of viscosity reducer got better. Oil displacement of viscosity reducer got worse with the increasing of curde oil viscosity, so the recovery amplification can be increased through preprocessing on viscosity reduction before similarty viscosity reducer was used.