In order to reveal the pattern of oil–water movement during water flooding development in ultra-low permeability reservoir, flow units division and multiple methods such as rate-controlled mercury injection, nuclear magnetic resonance, water flooding in real sandstone model, permeability test are used to perform quantitative characterization of ultra-low permeability reservoir from two aspects, geological property and coupling micro-mechanism. Radius size and number-percentage of mainstream throats and moveable fluid saturation for different clusters of flow units have been characterized. Displacement types for different clusters of flow units and remaining oil distribution types have been specified. This provides a geological evidence to develop and adjust the water injection development plan. In this paper, the number-percentage of mainstream throats has been first proposed, which is a critical parameter to evaluate the difficulty of the predominant channel development process during water injection development. The quantitative characterization of the dominant seepage channel is realized.