The oilfield sewage is usually re-injected in chemical flooding units and the water quality has a significant influence on the performance of chemical flooding. In this paper, the variation of injection water over years in Shengli oilfield is statistically analyzed.Our results show that the total salinity and divalent-cation (Ca2+ and Mg2+) content keep increasing, meanwhile, the impurities such as Fe2+, S2- and COD are contained in the injection water. The influence of water quality on the viscosity and interfacial tension is investigated. Experimental results show that the viscosity of polymer decreases due to the interaction between ions and polymer. The interfacial tension increases because the Shengli petroleum sulfonate (SLPS) precipitates with divalent cations. Furthermore, the viscosity at wellhead is much lower than that of measured in laboratory, implying that the chemical concentration used in the field has to be considerably increased in order to reach the same viscosity.