Abstract: The oily substances in the water produced by polymer flooding in Gudao, Shengli Oilfield were extracted by petroleum ether and trichloromethane and two fractions (marked as F1 and F2) were obtained. The acidic components from fraction F2 were separated by alkaline extraction and esterified by methanol. The methyl esters products were separated into three fractions I, II and III by silica gel column chromatography. The oily substances and the acidic components were characterized using elementary analysis, IR spectral analyses and acidimetry. The structures and compositions of the methyl ester fractions (fraction I) were identified using GCMS technology. The results showed that the acidic emulsifying active substances in the produced water by polymer flooding were gathered in CHCl3 extracted fraction F2, and degradation products of hydrolyzed polyacrylamide (HPAM) constitute the major part of the acidic components. The compositions of fraction I and II were mainly petroleum acid methyl esters and aliphatic acid methyl esters with amide groups. The fraction III was mainly composed of aliphatic acid methyl esters with more amide groups and some strong polarity compounds containing SO group. The light petroleum acids in water produced by polymer flooding in Gudao Oilfield were mainly composed of C16~18 aliphatic acids, C14~18 naphthenic acids and C18~21 tetracyclic naphthenic acids, and the amount of naphthenic acids were more compared with the aliphatic acids.