Abstract: To investigate how casing damage in injection wells affects the distribution of remaining oil during the polymer flooding stage in mature oilfields, complex potential theory was integrated with field streamline simulation and numerical simulation to establish a seepage model for well groups with casing damage. By comparing normal well groups, injection-controlled well groups, and casing-damaged well groups under different shut-in durations, the mechanisms by shut-in due to casing damage and injection control affect injection-production streamlines and the areal and vertical distribution of remaining oil were analyzed. The results show that casing damage during the polymer flooding stage disrupts the injection-production well pattern, producing sparse and deflected streamlines and reduced pressure gradients, which in turn lead to the formation of displacement blind zones and remaining oil enrichment zones. The impact of casing damage on the distribution of remaining oil is particularly pronounced in medium- and low-permeability layers. During the polymer flooding stage, the remaining oil saturation in casing-damaged zones was 0.032 higher than that in normal zones, and during the subsequent water flooding stage, this difference decreased to only 0.003. Based on numerical simulation, optimized combined measures for comprehensive well pattern adjustment and oil displacement scheme design were proposed. Field application demonstrates that maintaining the original polymer flooding well pattern while implementing the polymer injection system increases the recovery rate by 3.59 percentage points. The research results provide technical support for the remaining oil distribution and the formulation of potential tapping measures in casing-damaged well groups of mature oilfields.