The effects of cationic polymer concentration,cationic degree,and molecular structure on oily sewage treatment in the polymer flooding oilfield were investigated by the measurement of zeta potential,turbidity,particle size,filtration time,etc..The results showed that zeta potential of sewage without HPAM residue was reversed by addition of small amount of cationic polymer,and the oil droplets possessed stable dispersion.HPAM as EOR residue was beneficial to the stable dispersion of oil droplets in sewage.After treated with cationic polymer,zeta potential,increasing extent of particle size and turbidity of sewage with HPAM residue decreased with the increase of the cationic polymer dosage and cationic degree.The optimum dosage of cationic polymer with cationic degree ≥15%,decreased with the increase of the cationic degree.The degreasing efficiency of cross-linked polymer was better than that of linear polymer.After treated with cross-linked polymer,the filtration time of sewage with HPAM residue was shorter.