Polymer viscosity reducer shows a good application prospect in the recovery of conventional heavy oil reservoirs.The basic properties and emulsifying viscosity reduction properties of partially hydrolyzed polyacrylamide（HPAM）and two polymer viscosity reducers（P-OVR-1 and P-OVR-3）were compared in this paper. The contributions of their apparent viscosity and activity to enhanced oil recovery during the displacement were studied in depth. The results show that P-OVR-1 and P-OVR-3 have good emulsifying viscosity reduction properties for heavy oil，mainly through emulsifying and dispersing the crude oil. The smaller the particle size of the emulsion comes with a better viscosity reduction effect. Under the effects of increasing water-phase viscosity and reduced oil-phase emulsifying viscosity of P-OVR-1 and P-OVR-3，the oil recovery in large pores and small pores was increased in varying degrees. The enhanced oil recoveries of large and small pores after P-OVR-3 flooding were 8.48% and 9.59% respectively. The residual oil in small pores was mainly obtained by the viscoelasticity of polymer viscosity reducers. In large pores，the efficient displacement of columnar residual oil and oil film was realized mainly depending on the high surface activity of polymer viscosity reducer to reduce the capillary force，adhesion and cohesion.