There have been numerous laboratory experimental studies and field tests on ASP flooding. Most of the laboratory experiments focused mainly on the effect of the ultra-low oil-water interfacial tension on oil recovery. The relationship between the viscosity of the ASP solution and oil recovery efficiency of ASP flooding has not been well understood. In this paper, the relationship between the viscosity of an ASP solution and oil recovery in heterogeneous porous models was studied. More than 50 ASP flood tests were conducted using artificial models. These models were made to have different permeability variation coefficients so that ASP floods in homogeneous and heterogeneous reservoirs could be simulated. Sixteen ASP solutions were prepared and used in ASP flood tests. Based on the results of ASP flood tests, the effects on tertiary oil recovery of NaOH concentration and of the balance between the IFT reduction and viscosity increase were discussed. For heterogeneous models, it was found that there existed a minimum viscosity value of ASP solution for ultra-low IFT systems to fully work towards improving residual oil recovery. This minimum viscosity is defined as critical displacement viscosity in this study. When the viscosity of an ASP solution is lower than the critical displacement viscosity, the oil recovery efficiency of ASP flooding is dominated by the viscosity of the ASP solution. The reduction in interfacial tension to an ultra-low level contributed little to oil recovery. When the viscosity is higher than the critical displacement viscosity, both the viscosity and ultra-low IFT contributed to oil recovery. This critical displacement viscosity should be one of the important parameters that can be used to optimize the chemical formula of an ASP flood for a target reservoir.