Viscosity loss occurs in polymer flooding in middle and high permeability reservoir. For larger specific surface and more clay in low permeable rock, the actual viscosity of polymer solution is more important. Polymer cores’ flow experiments are reported which were performed using long natural geological outcrop sands cores. These cores were in good simulation of natural cores and the length of cores allows viscosity variation mechanisms that occur when polymer solution flows through long distance. These mechanisms can not be assessed by silica cores of conventional length. Results of polymer cores’ flow experiments gives viscosity of polymer solution after it through different permeability cores, and the effects of molecular weight, concentration of polymer, migration distance, permeability, and injection velocity on viscosity loss rate. Results show the molecular weight and concentration of polymer is higher, or the core’s permeability is lower, and the viscosity loss of polymer solution through cores is greater. Injection velocity is found to be insignificant within 15m/d for polymer with middle or low molecular weight. This simulation work confirms the viscosity loss mechanism for one polymer in low permeable cores.