<p>After years of waterflooding development, Block X of the Shengli Oilfield has predominantly entered the ultra-high water-cut stage, making production stabilization the primary objective for subsequent development. For mobilizing residual oil in weak water-flushed zones of heterogeneous reservoirs, an iterative calculation method was developed to estimate the advancement distance of the waterflood front in low-permeability layers based on Darcy’s Law and the Buckley–Leverett non-piston displacement theory. This method was further extended to polymer flooding. A numerical simulation model was built and history-matched to confirm that the model met the required accuracy for comparison and evaluation. Using the proposed method, the flow resistance of heterogeneous formations with different permeabilities after polymer slug injection was calculated. In addition, the effectiveness of polymer slug injection at different water-cut stages was predicted. The study also analyzed the impact of injecting different volumes of polymer slugs—under identical timing conditions—on the mobilization of residual oil in weak water-flushed zones during subsequent waterflooding. Furthermore, a comparative analysis was conducted to evaluate the performance of polymer flooding across reservoirs with different permeability contrasts under the same injection scheme. The proposed calculation and prediction method were validated through comparison with numerical simulation results, demonstrating good applicability and accuracy in evaluating the mobilization status of weak water-flushed zones during polymer flooding. This approach enables efficient and quantitative assessment of polymer flood effectiveness in starting the weak water-flushed zone. By applying this iterative prediction method in conjunction with numerical simulation, the study provides an optimized development strategy for the transition from waterflooding to polymer flooding and subsequent waterflooding in Block X of the Shengli Oilfield.</p>