The effects of various factors on viscosity loss of polymer solution are investigated to highlight the main issue of serious loss for viscosity during foam flooding as well as to find the effective preserving measurements. The results showed that the reducing substances in the wastewater such as sulfides, Fe2+, SRB, and COD are the main reasons affecting the viscosity loss of polymer solution. On this basis, one kind of preserving method for viscosity loss of polymer solution in Chengdong foam flooding is pointed out by aerobic biological oxidation. The viscosity of polymer solution prepared with the wastewater which is aerobically treated by sulfur-oxidizing bacteria SOB and denitrifying bacteria DNB can reach more than 30 mPa?s.