In the cold-chamber high-pressure die casting (CC-HPDC) process for light alloys, strong shear stress generated by the fast-flowing melt through narrow runners breaks externally solidified crystals (ESCs). Two runner configurations were applied in the CC-HPDC process of aluminum alloy to address this problem. A comprehensive finite element model was established to calculate shear stress in the runner regions during die filling, and a novel mathematical model of grain breakup was proposed to quantitatively analyze ESCs fragmentation through different runners. Particles ranging in size from 12.2 to 16.1 μm constitute a significant proportion of the ESCs and serve as the primary focus of subsequent shear fragmentation. Finally, HPDC test trials validate the mathematical model by characterizing grain morphology and size distribution in as-cast samples and the error of the model is less than 20%. The results demonstrate that the novel model is highly effective for the design of runner systems and the optimization of process parameters in the CC-HPDC process for light alloys.