Abstract: The relationship of polymer molecular structure and long-term stability was studied in the third oil reservoirs condition in Shengli Oilfield. The results indicated that hydrophobically associating polymer has best long-term stability performance depending on interaction between molecule chains, then the worse is heat and salt resistance copolymer, and the worst is polyacrylamide with high molecular weight. Through improving hydrophobic monomer content, enhancing polymer molecular weight, reducing the degree of hydrolysis and introducing the function monomer in the molecular structure, the long-term stability performance has been improved obviously. When hydrophobe molar fraction is 0.5%, 2-acrylamido-2-methylpropane sulfonic acid quality score is 15% in the hydrophobically associating polymer molecular structure, the intrinsic viscosity value is more than 1 400 mL/g and the degree of hydrolysis is lower than 20%, the solution will not occur phase separation and still has certain viscosity ability after aging 120 days.