<p>Shrinkage cavities and macrosegregation, typical metallurgical defects in continuous casting billets, affect the quality of finished steel products. Currently, adopting an appropriate mechanical reduction control strategy can effectively mitigate these defects in continuous casting billets. Considering the thermal shrinkage, solidification shrinkage, and deformation behavior of the solidified shell, a multiphase solidification model was established in this work to investigate the effect of mechanical reduction on macrosegregation and clarify the formation mechanism of shrinkage cavity defects in continuous casting billets. The model was validated against experimental data from SWRH82B steel continuous casting billets, with a focus on comparisons of microstructure and macrosegregation. Furthermore, the study analyzed the impact of mechanical reduction on melt flow and macrosegregation, clarified the formation mechanism of shrinkage cavities, and explored optimal positions for implementing mechanical reduction to control defects. Results indicate that mechanical reduction applied only in regions with low solid-phase volume fractions (<i>g</i>_s ≤ 0.8) is insufficient to fully alleviate macrosegregation in continuous casting billets. By contrast, when the reduction region covers zones with a solid-phase volume fraction between 0.9 and 1.0, mechanical reduction can effectively inhibit the rapid solute enrichment before complete solidification, thereby further reducing central macrosegregation. In addition, internal shrinkage cavities in continuous casting billets form primarily after complete solidification, driven by thermal shrinkage. Implementing mechanical reduction at the solidification end after the billet has fully solidified is thus a critical measure for controlling internal shrinkage cavities. These findings provide theoretical guidance for optimizing continuous casting process parameters and improving billet metallurgical quality.</p>