Polymer flooding is an important tertiary recovery method. However, even after its application, more than 50% of the oil remains in the formation layers. It is thus important for efficient resource use that both residual oil distribution and the mechanisms associated with polymer flooding are better understood. In the work described in this paper, a series of in situ X-ray tomography experiments were carried out, and different displacement stages were imaged. Then, rock, water, oil, and polymer were identified in the imagery and quantified, while interchanges among the different types of residual oil, and their associated microscopic polymer displacement mechanisms, were discussed. We also studied the impact of permeability on polymer flooding efficiency, and our results showed that polymer flooding could mobilize residual oil that was not accessible via water flooding. Mechanistically, the polymer flood caused the big continuous residual oil clusters to split, dispersing them into smaller clusters. The polymer also changed fluid flow pathways, thus increasing sweep efficiency, while its viscoelasticity was helpful for recovering singlet and film residual types of oil. We found that the polymer could only drive oil locally, and that a large amount of residual oil remained in the middle of the core. We also noted that polymer flooding was more effective for higher permeability samples, which showed that, after polymer flooding, the recovery rate for the samples with permeabilities of 3989, 1543, and 814 millidarcys (mD) were enhanced by 10.13, 6.30, and 4.34%, respectively. Higher permeability samples remained as network-type and multiple-type residual oils after polymer flooding, while samples with lower permeability showed multiple-type and singlet-type residual oils. Thus, our results and conclusions showed that different enhanced oil recovery methods could be applied for further development.