Permeable viscosity reducing displacement agent is a polymer viscosity reducing agent，which has excellent effects of water-phase viscosity increasing and oil-phase viscosity reducing. It can inhibit water channeling and effectively improve the recovery of heavy oil by water flooding. From the mesoscopic and microscopic levels，the mechanism of enhanced oil recovery by permeable viscosity reducing displacement agent had been analyzed and explored. CT scanning oil displacement experimental study shows that in the process of simulating heterogeneous reservoir flooding，2 peaks of pure oil phase and emulsion phase appear，and the equilibrium utilization of high and low permeability layers is realized. The results of micro displacement model displacement experiments show that permeable viscosity reducing displacement agent can form many kinds of fluid forms with heavy oil during the displacement process and play different roles in the process of reservoir flooding：the formation of oil-in-water（W/O）emulsion at the initial stage of displacement will enlarge the swept volume. Continue to inject the agent，W/O emulsion can be converted into water-in-oil-in-water（W/O/W）emulsion to improve flow capacity. The formation of micro emulsion can improve the stripping efficiency of oil phase on rocks. The distribution characteristics of remaining oil after displacement by permeable viscosity reducing displacement agent show that the relative content of cluster flow and drop flow is significantly reduced，indicating that it has excellent sweep expansion and oil washing effect，which has important reference significance for remarkable improving the recovery of low efficiency water flooding heavy oil reservoirs.