<p>To mitigate the precipitation of large skeletal primary carbides during the solidification of GH3128 superalloy, this paper investigates the effects of cooling rate (10 °C/min to 30 °C/min) on the microstructure, segregation, and primary carbides of both Mg-free and 0.018-wt pct Mg-containing GH3128 superalloys based on high-temperature confocal laser scanning microscopy experiments. The results show that both increasing the cooling rate and adding Mg refine the dendritic structures. This refinement shortens the diffusion distance of solute elements and reduces their segregation, thereby inhibiting the growth of primary carbides and decreasing their size. Simultaneously, the refined dendritic structures provide limited growth space for the primary carbides, restricting their growth and aggregation, which promotes their refinement, ensures uniform dispersion, and increases their quantity. Additionally, to effectively acquire the solute elements necessary for growth, the primary carbides are forced to transform from skeletal shapes into blocky and elongated shapes. The synergistic effect of increasing the cooling rate and adding Mg amplifies the impact on the primary carbides. Compared to the Mg-free superalloy at a cooling rate of 10 °C/min, the average diameter and area fraction of primary carbides in the 0.018-wt pct Mg-containing superalloy at 30 °C/min are reduced by 80 pct and 71 pct, respectively.</p>