Abstract: In order to reveal the reservoir physical property changes in water flooding for offshore heavy oil sandstone reservoirs, systematically study has been carried out taking the Z Oilfield in Bohai Sea as an example. The data of clay minerals, pore structure, porosity, permeability and seepage characteristics are fully applied, in addition to multi-core wells analyses and actual production data. The researches show that after long-term water flooding, the total amount of clay minerals shows an obvious decreasing trend. The kaolinite content is decreased and the content of mixed layer illite/smectite increased. As the consequence, the average pore radius, the pore throat and pore structure increased, and the average porosity and permeability increased to varying degrees. The increase in porosity is little, but in permeability is obviously improved. The overall physical properties of reservoir become better, but the degree of heterogeneity is enhanced. Reservoir wettability is characterized by greater hydrophilicity and increased oil displacement efficiency. The research can effectively guide the adjustment of the development of remaining oil and injection-production structure in high-water-cut period of the Z Oilfield.