Water drive characteristic curve is widely used in the middle-high water cut development stage. However，when oilfield entered extra high water cut period，the water drive characteristic curve will slope upward and an inflection point will occur. Focusing on this phenomenon and according to124 water flooding experiments with high multiple injection （1 000 pore volume），it was found that the fundamental reason for the upward water drive characteristic curve is due to the sharp increase in water permeability，and it was pointed out that the inflection point is an objective existence for the water drive characteristic curve. At the same time，affected by the cumulative effect of water drive characteristic curve，the inflection point can only be found after its appearance，resulting in a delayed estimation on its occurring time. In order to eliminate the influence of the cumulative effect of water flooding characteristic curve on the inflection point estimation，a lgfo-Np formula was derived starting from the first water drive characteristic curve. The cumulative impact of water drive characteristic curve can be overcome by application of this formula to estimate the accurate occurring time of the inflection point.The new method can help to make technical development strategies in time，which lay the foundation for the field to improve the development effect of oilfield production and to guide the practice better.