Abstract: Oil and water distribution within the reservoirs in mature oilfields with extremely high water cut is very complicated. To tap the development potential of such fields, it is necessary to have a clear definition of the remaining oil, its attributes and classification method, and the approaches for evaluating the potential for further developing the fields with a waterflooding program. A "Four Points and Five Types" remaining oil classification method for oil fields with ultra-high water cut was developed in this paper by selecting 4 oil saturation points corresponding to the oil-water seepage characteristics as the boundary points of remaining oil classification, based on the investigation of the ultimate oil displacement efficiency and the analysis of the seepage mechanics. The remaining oil in fields with ultra-high water cut was classified into five types, including enriched oil, somewhat enriched oil, movable oil, hard-to-move oil, and residual oil. The specific features and re-development strategies of these five types of remaining oil were analyzed. Based on the presented method for evaluating the waterflooding development potential of oilfields, the waterflooding development potential of oil fields with extremely high water cut was categorized into three types, namely, ultimate waterflooding potential, movable potential and hard-to-move potential. Investigation of the remaining oil types and waterflooding development potential of 10 fields with mid-high permeability showed that majority of the remaining oil in fields with extremely high water cut was hard-to-move oil, and there was rather enriched oil in some local areas. Although there was huge waterflooding development potential in such fields, nearly 4/5 of the remaining oil was hard to produce. In summary, the "Four Points and Five Types" remaining oil classification method for fields with extremely high water cut helped to make clear the remaining oil distribution features and provided supports for working out re-development strategies of the various types of remaining oil.