Abstract: Regarding the complex microscopic pore structures and the weak understanding on water flooding regularity, the paper studies the microscopic pore structures and the flowing characteristics of oil displacement by water in the reservoir of the Chang-8 member by using sandstone waterflooding model, cast thin section and SEM analysis in Zhenbei oilfield, Ordos basin. The results show that the accumulation spaces of the Chang-8 reservoir in the study area can be divided into residual intergranular pores, dissolved pores and microcracks; the displacement types are dominated by network displacement and finger-like displacement and even displacement seldom occurs. Different types of waterflooding have different displacement efficiencies, the efficiency of even displacement is the highest and that of finger-like displacement is the lowest. The waterflooding efficiency highly correlates with the permeability with the correlation coefficient of 0.84 and the correlation coefficient between the waterflooding efficiency and porosity is 0.51. Reservoir heterogeneity is also an important factor influencing the waterflooding efficiency.