Abstract: Water flooding microscopic seepage mechanism was researched by stochastic simulation on percolation network model at microcosmic level. The validity of network model was testified by the comparison of simulation and experiment of water-oil relative permeability steady flooding. Then, the microscopic distribution laws of remaining oil were discussed at different stages of water flooding and at different wetting situations. The forms of remaining oil distribution were divided into single grain (or single drop) shape, fleck shape, network shape and oil-water mixed shape. Research results show that the blocks of network shape remaining oil are less than other forms while occupying more volumes. Pores with max network shape remaining oil are reduced together with the reducing of remaining oil saturation and are reduced rapidly at about forty percent to fifty percent of remaining oil saturation. Wettability not only influences sweep efficiency, but also influences remaining oil distribution. The general change tendency of remaining distribution is becoming more and more dispersive.