Abstract: Hydrolyzed polyacrylamide (HPAM) is a kind of polymer used for displacement of reservoir oil, while the oilfield sewage used to prepare HPAM may reduce its viscosity and impact the displacement efficiency. In this work, the HPAM solution was prepared according to the metal ions composition in Chengdong Oilfield sewage and the effect of metal ions on the viscosity of HPAM solution was investigated. It was found that the influencing extent of various metal ions on the viscosity of HPAM solution is in the order of Na⁺ > Fe²⁺ > Ca²⁺ > K⁺ > Mg²⁺. IR and SEM characterization results suggested that Na⁺, K⁺, Ca²⁺ and Mg²⁺ interact with the carboxylate anions, which may decrease the HPAM molecular charge density, make the polymer curl, reduce the solvation ability of polar groups, and release a large number of "bounded" water; these lead to the degradation of HPAM solution viscosity. On the other side, the viscosity degradation by Fe²⁺ is attributed to its interaction with oxygen in sewage, which initiates a free-radical reaction and causes the disintegration of the polymer chains.