Molecular weight of Polymer is an important parameter to evaluate the injectability of chemical flooding system，which directly affects the field injection pressure and oil displacement effect. In view of the problems of high injection pressure，poor injection and uneven effect of individual well groups in the chemical flooding pilot test area of block S of medium low permeability high pour point oil reservoir，the matching relationship between polymer molecular size and reservoir micro pore throat after cold damage of high pour point oil reservoir was studied by using dynamic light scattering method，microporous membrane method and natural core injectability experiment. Based on the polymer molecular radius of gyration，hydrodynamic size and injectability test results，the polymer molecular weight in block S was adjusted from 2000×10⁴ to 1200×10⁴ ，and the injection thickness ratio of single well was increased from 42%—51% to 64%—85%，which provided a basis for polymer molecular weight adjustment in the pilot area and decision-making in the expansion area.