Abstract: Alkali/surfactant/polymer(ASP) flooding was taken as an example, and the synergistic effect of interfacial and rheologic property(SEIRP) of displacing fluid on enhancing oil recovery has been studied experimentally and theoretically. The results show that the higher the visco-elasticity of the displacing fluid, the stronger the flow disturbance towards bypassed area in pores, and the higher the oil recovery efficiency. Visco-elasticity is beneficial for displacing residual oil. In experiments on natural cores, when the equivalent viscosity of the displacing fluid decreased to a certain value(Critical Viscosity), decreasing the interfacial tension of water/oil cannot improve significantly the displacement efficiency, and the idea of SEIRP was presente. In artificial heterogeneous cores, the effect of the interfacial and rheologic property of the ASP solution on the oil recovery efficiency was non-monotonic, and exhibited a maximum value. When the equivalent viscosity was lower than the critical viscosity, even the interfacial tension of the displacing fluid was reduced greatly, the oil recovery efficiency also decreased with the decrease of the equivalent viscosity. Under the experiment conditions, the critical viscosity was about 20 mPa·s. The oil displacement ability of the chemical displacing fluid depend on the SEIRP. Therefore, under a certain reservoir condition, it is more important to optimize the SEIRP than to make excessive demand for the ultra low interfacial tension. The results are helpful for optimizing the composition of displacing fluids.