Old oilfields in China have successively entered into middle to late stage of oil production. Affected by strong reservoir heterogeneity，large amounts of remaining oils are still in the subsurface and provide an important material foundation for a long-term stable yield in old oilfields. It is a highly integrated and difficult research problem to predict potential areas of remaining oil under high water cut condition. The theoretical and practical prediction have never ceased but with fewer researches on enrichment mechanism of remaining oils. Research on oil-water flow rule is the key of water flooding in the low-permeability reservoirs. Oil-water rate difference and capillary force is not considered in Buckley-Leverett principle. Based on the theory of fluid potential energy and petroleum migration and accumulation，the formulae of oil-water seepage velocity and reservoir forming velocity of remaining oil re-enrichment in the whole fluid migration course of water flooding were deduced first in consideration of two states，oil in water and water in oil，in water-wet reservoir by means of force analysis using Darcy’s Law. The reservoir forming mechanism of remaining oil re-enrichment at different develop? ment stages in the water flooding reservoir was proposed. The research results show that the hydrodynamic force has a big impact on the remaining oil re-enrichment velocity in water flooding reservoir，and the remaining oil re-enrichment velocity increases with the increase of air permeability，water saturation and oil-water density difference，and it decreases with the increase of hydrodynamic pressure gradient，crude oil viscosity and dip angle. The field application indicates that the mechanism of remaining oil re-enrichment in the water flooding reservoir is reasonable and practical.