Abstract: Polymer solution prepared by Oilfield sewage resulted in significant viscosity reduction, influencing the application efficiency and oil recovery effect seriously. Based on the foam combination flooding test in Chengdong Oilfield and the analysis of reservoir geology, surface flow and sewage quality, a study was conducted about effect of sewage on polymer solution viscosity. The results showed that the viscosity loss of polymer solution mainly occurred when polymer mother liquid mixing with sewage. The main reason was that the redox reaction of reducing substance in sewage and dissolved oxygen occurred when trace metal ions were present, which caused polymer chain scission, and a sharp drop of viscosity. The main countermove is to eliminate or decrease the reducing substance and trace metallic ions before the sewage contacts with polymer mother liquid. The results can be used to guarantee the quality and effect of field polymer injection.