Abstract: The pore structure of ultra-low permeability sandstones is highly complex, with strong wettability heterogeneity, which complicates the characterization of oil-water flow behavior and remaining oil distribution during waterflooding. Current research methods mainly rely on core-scale experimental analysis, which falls short in accurately capturing pore-scale dynamic flow processes in heterogeneous porous media. As a result, the microscopic mechanisms of oil displacement remain inadequately understood. To address this, visual models based on real core samples from ultra-low permeability reservoirs in the Ordos Basin were constructed to conduct waterflooding experiments. Numerical simulations were also employed to investigate the characteristics of oil-water flow and distribution during waterflooding. The results demonstrate that waterflooding in ultra-low permeability rocks exhibits a fingering pattern. Both a reduction in pore throat radius and an increase in heterogeneity lead to decreased oil displacement efficiency. Increasing the injection rate improves oil displacement efficiency in oil-wet rocks by up to 17%, whereas in water-wet rocks, it first increases and then decreases, peaking when viscous and capillary forces reach equilibrium. As rock wettability shifts from strongly water-wet to strongly oil-wet, oil displacement efficiency decreases by 20.2%, and the dominant forms of remaining oil transition from corner and throat types to clustered and interconnected types. Under mixed-wettability conditions, water phase flow paths alter significantly, with the proportion of clustered remaining oil being 17.2%–33.4% higher compared to purely oil-wet or water-wet systems. This leads to lower sweep efficiency and displacement efficiency relative to homogeneous wettability systems. Due to capillary effects, the wettability at the pore inlet plays a decisive role in controlling fluid flow under mixed-wettability conditions. This study provides an important theoretical foundation for the effective development of water injection in ultra-low permeability reservoirs. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202506030140.