In order to study the oil displacement effect of compound of different molecular weight polymers，core flooding experiments were carried out by using on-line nuclear magnetic resonance（NMR）imaging technology with 10 million single molecular weight polymer and compounded polymer. According to the NMR T2 spectrum signal after water flooding stage and polymer flooding stage，the quantitative characterization of the recovery proportion in pore throats of different sizes was performed and analyzed. The results show that by using polymers with same viscosity and cores having similar pore structure，the recovery and the proportion of fluorine oil recovered from the medium-and small-sized pore throats to total oil by compounded polymer are respectively 1.5% and 7.1% higher than those by single molecular weight polymer. The small molecular weight polymer in the compounded polymer has a small volume and can effectively displace the fluorine oil in the medium-and small-sized pore throats，while the large molecular weight polymer also has good displacement effect in large pores. Therefore，compared with the single molecular weight polymers，the compounded polymer can drive oil in a wider pore throat size range，and ultimately achieve better displacement effect.