The main uncompartmentalized oilfields in Shengli Petroleum Province have entered the late waterflooding stage after long-term development. Their comprehensive water cut has reached more than 98%，but the recovery rate is only 41%，and thus the oilfields still have large residual oil reserves and enhanced oil recovery（EOR）potential. Through the core analysis of cored wells in uncompartmentalized oilfields，it was found that in the ultra-high water cut stage，there were extreme waterflooding intervals with the saturation of remaining oil less than that of residual oil as well as weak waterflooding intervals with the oil saturation greater than the oil saturation of waterflood front in all uncompartmentalized oilfields.Therefore，this paper proposed the definition and classification criteria of the extreme waterflooding zone，strong waterflooding zone，and weak waterflooding zone of an ultra-high water cut reservoirs. The influence of water injection，displacement rate，and permeability on the formation of the extreme waterflooding zone was studied. The research results reveal that with the increase in injected water volume，in the priority breakthrough area of the injected water，oil saturation gradually decreases with the gradual rise in water phase permeability，water fractional flow，and displacement rate. As a result，the remaining oil saturation in this area is lower than the residual oil remaining oil saturation，and thus the extreme waterflooding zone is formed. The volume calculation method for different graded waterflooding zones was established，and the dynamic production data of Gudong，Gudao，and Chengdong oilfields were used to verify the calculation. It is proved that the established method is scientific and accurate and has the advantage of easy access to required data and simple calculations.