The invention discloses a viscosity-reducing oil displacement agent for heavy oil reservoirs, wherein the components comprise, by weight, 20-60% of an emulsifier, 1-5% of an ester-based compound, 0.2-2% of a wetting agent, and the balance of water. The invention further discloses a viscosity-reducing oil displacement agent-polymer system for heavy oil reservoirs, wherein the viscosity-reducing oil displacement agent-polymer system comprises 0.1-1% of the oil displacement agent, 0.05-0.3% of a polymer and the balance of water, and the polymer is polyacrylamide with a molecular weight of 8000000-20000000. According to the present invention, the viscosity-reducing oil displacement agent and the viscosity-reducing oil displacement agent-polymer system can provide good effects of viscosity reducing and emulsifying for heavy oil, can substantially reduce the viscous force of heavy oil in rock pores, can provide good profile control and water blocking effect, can effectively improve oil displacement efficiency, can improve heavy oil recovery, and are suitable for heavy oil reservoirs with a stratum viscosity of more than 200 mPa.s.