Abstract: In order to improve the polymer flooding effect in high-temperature and high-salt Class III reservoirs in the Shengli Oilfield, the solution characteristics and oil displacement effect of a temperature and salt-resisting ultrahigh molecular association polymer AP-P5 are studied. Basic physicochemical properties of AP-P5 are evaluated under Class III reservoirs at 85 degrees and salinity of 32 868 mg/L. The viscosity at varied concentrations is examined, the hydrodynamic radius distribution of the solution is determined and the adsorption is studied through mixed polymer solution and quartz sand. Finally, the injection and seepage characteristics and oil displacement effect are investigated through physical experiments. A field test is carried out in the Tuo 28 Block of Shengli Oilfield to analyze the injection of AP-P5 solution and production after injection. The results show that AP-P5 possesses double molecular weight, smaller hydrodynamic radius and no significant critical associating concentration comparing with ordinary associating polymer DH3. Moreover, it is lower by 58% in the associated monomer weight, 30% in adsorption capacity and 50% in injection pressure and higher by 3.9% in the recovery rate. More field tests are ongoing but it has not shown water reduction and production increasing effect. The research reveals that AP-P5 has good solution properties and it is easier to generate a piston-like movement with it than DH3 in formations;further, it has higher displacement efficiency.