Abstract: Using the value b/a in type A water drive curve, the paper studies the distribution features of the remaining oil in the late waterflood reservoir, and the result was applied in bottom water drive reservoirs in Daiyi segment of Subei CS oilfield and Duoyi segment of XNZ oilfield. This result is equal to the reservoir simulation result. The remaining oil zone in those two reservoirs was tested by slim-hole milled window in casing sidetracking directional well and the developing adjustment wells. The oil rate of new well is more than twenty tones one day. The application of the method shows that the method is practical and simple in looking for the remaining oil of fault block reservoir.