Abstract: Through more than 20 years' water flooding development, the microscopic characteristics of reservoirs in Wangyao Area of Ansai Oilfield have been changed in some degree due to long-term water flooding and washing. In order to investigate the changing mechanism of formation characteristics, inspection wells are deployed in different water flood areas, and sealing core data from these inspection wells are analyzed. Results have shown that reservoirs which are strongly watered out have coarse granularity, high surface porosity and good pore throat structure. Besides, neogenic calcite which is obviously different from the original mineral is generated in these intervals. Compared with the initial formation before water driving, physical properties of water flooded reservoirs get worse because of the migration of clay mineral and generation of new minerals. The effects of water flooding mainly show up in the increased median pressure of reservoir throat during the middle and late stage of water flooding. There is lower mercury withdrawal efficiency and poorer relative permeability curve, which decreases the displacement efficiency of water flooding.