<p>In the present work, the impact of riser geometry and its thermal insulation performance on macrosegregation and the Columnar-to-Equiaxed Transition (CET) position of a 5.5-ton steel ingot was investigated using a three-phase mixed columnar-equiaxed solidification model. The model fully incorporates the competitive growth of microstructures, mass transfer, heat transfer, fluid flow, and dynamic migration of equiaxed grains, etc. The accuracy of the three-phase model was verified through sectioning experiments on a 5.5-ton steel ingot. The results show that for steel ingots with a fixed riser mass ratio, a negative taper riser is conducive to alleviating central positive segregation and reducing the range of bottom negative segregation. As the riser taper increases, the length of the A-type segregation band gradually shortens, and its severity decreases. Increasing the riser mass ratio not only enhances the equiaxed grain density in the ingot core but also effectively mitigates the severity of macrosegregation. In addition to the optimized design of riser geometry, enhancing riser thermal insulation can significantly suppress the occurrence of A-type segregation and improve negative segregation in the middle and lower sections of the ingot. Furthermore, different riser designs cause varying degrees of offset in the CET position, but minor modifications to the riser have a rather limited impact on the CET position. These changes are primarily attributed to the fact that different riser designs significantly alter the temperature gradient during ingot solidification, thereby influencing microstructural evolution and solute distribution. This study provides theoretical guidance for the riser design of steel ingots from the perspective of inhibiting macrosegregation.</p>