Abstract: The outstanding problems of water injection wells in an oilfield in Bohai are high injection pressure and shorter validity acidification. Combining the geologic feature of reservoir and the waterflooding development history, the subjects were systematically studied, such as the compatibility of mixed fresh water and polymer-contained sewage, the components of suspended matters, the microstructures and molecular size of HPAM, the compatibility of injection water and cores, etc. Researches show that suspended solids jam is the main damage forms. Suspended matters are main the flocculation products of the produced polymer with the wastewater treating agents and Ca²⁺, Mg²⁺ bivalent cationic, etc. Secondly, they also contain the precipitation of formation fines and produced polymer. Meanwhile, the suspended matters include a few precipitation of CaCO₃ and FeCO₃. In the early time, serious water sensitivity happened caused by fresh water injection. Rigid granules like CaCO₃ and FeCO₃, the precipitation of mixed fresh and produced water for reservoir flooding, intensified the obstruction level of formation in wellbore area nearby, the flocculating-deposition of produced polymer and other products blocked the deep area of formation, which were the main reasons for the poor waterflood effect and less effective acidification.