Abstract: Fluid mobility in tight reservoirs is an important indicator for assessing the potential and difficulty of reservoir development. To quantitatively analyze the distribution characteristics and utilization patterns of the fluid in the tight reservoir of Chang 8 oil layer in Longdong area of Ordos Basin, a combined method of water flooding experiments and nuclear magnetic resonance scanning was adopted to test the core samples from the reservoir and analyze the distribution patterns and utilization characteristics of the fluid in the pore throats with various scales. The results show that the reservoir pore throats in the study area are predominantly composed of micropore smaller than 0.1 μm and small pores ranging from 0.1 to 1 μm. The saturation of the mobile fluid is significantly affected by the pore throat structure. The samples dominated by micropores exhibit lower mobile fluid saturation. In the process of displacement, majority of the mobile oil is developed in the early and middle stages. As the injection pore volume increases, the displacement efficiency gradually decreases. Pore throats sized 0.1 to 1 μm are the main occurrence spaces of mobile fluids. With the improvement of reservoir physical properties, the proportion of micropores gradually decreases, but pore throats larger than 0.1 μm remain the main occurrence locations for mobile oil. This research provides theoretical support for the evaluation of Chang 8 tight oil reservoir and the optimization of waterflooding strategies in Longdong area of Ordos Basin.