To elucidate the freeze–thaw damage mechanism of asphalt-stabilized macadam from a mesoscopic perspective and to quantify the deterioration of internal parameters, this study examines the effect of adding 2% SBS to cold-region ATB-25. Specimens subjected to 0 to 25 freeze–thaw cycles were scanned using X-ray computed tomography (CT), and image-based morphometric analysis was conducted to track the evolution of key meso-scale indices, including porosity, void number, equivalent void diameter, and void fractal dimension. The results show that the porosity increase in modified ATB-25 (3.06% to 3.71%, approximately 21%) is markedly lower than that in conventional ATB-25 (3.31% to 4.31%, approximately 30%). Similarly, the increase in void number for the modified mixture (2,898 to 4,452, approximately 54%) is smaller than that for the conventional mixture (3,364 to 5,813, approximately 73%). The equivalent void diameter initially decreases and subsequently increases, whereas the void fractal dimension initially increases and then decreases, indicating early closure of small defects followed by void coalescence under cumulative freeze–thaw loading. These CT-based results demonstrate that incorporation of 2% SBS effectively restrains void growth and suppresses the formation of new voids, likely due to enhanced binder elasticity, improved aggregate–mastic bonding, and reduced moisture susceptibility. Consequently, the SBS-modified ATB-25 exhibits superior resistance to freeze–thaw damage. This study provides quantitative mesostructural evidence supporting the use of SBS-modified asphalt-stabilized macadam in cold regions and informs the optimization of mixture design to improve durability under cyclic freezing and thawing.