Aiming at the problem that there is no basis for selecting the molecular weight and concentration of polymer in the process of adopting the binary composite flooding in low permeability reservoir，the laser particle size analyzer is used to test the hydrodynamic dimensions of the binary compound flooding system under different conditions，and the change law of the hydrodynamic diameter of the binary compound flooding system with the molecular weight of polymer，the concentration of polymer，the concentration of surfactant and the mineralization degree of the base fluid was clarified. The average throat radius of cores with different permeability was tested by constant velocity mercury injection experiment，and the empirical formula of artificial core permeability and average throat radius was fitted. based on the injection test results of the binary flooding system and the judgment criteria calculated by the field data，it was clear that when the core permeability was less than 20 × 10^-3 μm² ，the molecular weight of the injected polymer should be less than 3 million and the concentration was less than 500 mg/L；when the core permeability was between 20×10^-3 —40×10^-3 μm² ，the molecular weight of the injected polymer should be less than 5 million. When the core permeability was between 40×10^-3 —70×10^-3 μm² ，the molecular weight of the injected polymer should be less than 12 million. The influence of the salinity of the binding base solution and the concentration of the surfactant on the hydrodynamic size of the binary system could provide guidance for the adjustment of the on-site binary system formula.