Abstract: As development of offshore oil field is limited by the life of platform, it is necessary to research on the proper time of polymer injection. Using plane physical model test and numerical simulation method, effect of polymer flooding at different injection time was investigated. Result of physical model test showed that for the field that had enough long life, there is no big difference between recovery of polymer flooding and water flooding and the recovery of polymer flooding at different time has also no big difference; but for the field that had limited life, injection time should be earlier. Result of numerical simulation showed that for offshore oil field, early polymer flooding can get more recovery increase, more daily oil production per ton polymer and more profit. Based on the research, early polymer flooding was implemented in well group A of A oil field, and preliminary analysis and evaluation on the effect of polymer flooding is carried out too.