Abstract: The Chang 8₁ reservoir of the Maling oil field in the Ordos Basin has low porosity and low permeability. The quality of oil and gas reservoirs is constrained by the strong heterogeneity of its permeability. We analyzed the characteristics of its microscopic pore structure by conducting an NMR experiment and microcosm experiments including constant speed mercury injection, high pressure mercury injection, image granularity, and casting lamella. We then divided the flow units into 4 classes E, G, M and P, using SPSS data analysis software on the basis of 5 chosen parameters, including sand thickness, porosity, permeability, oil saturation and flown zone exponent. We analyzed the microscopic pore structure characteristics in different types of flow unit and the impacts on movable fluid saturation. The results showed that there are obvious differences in the microscopic pore structure characteristics in different flow units, which are the main factors that caused different movable fluid saturation. The distribution and size of pore throat radius played a crucial role. Production performance data demonstrated that classes E and G acquired the highest capacity in the oil and gas field development. Reasonable and effective development programs should be implemented according to the microscopic pore structure characteristics of different flow units.