Driven by China’s carbon peaking and carbon neutrality goals, pumped-storage hydropower projects have developed rapidly, while power stations in Northwest China face challenges posed by extreme temperature conditions. Unlike previous studies on asphalt mixtures under conventional freeze–thaw conditions, this study employs multiscale characterization methods to investigate the deterioration behavior of dense-graded hydraulic asphalt concrete under extreme temperature cycling. SBS-modified asphalt concrete specimens were vacuum-saturated and subjected to 0, 5, 10, 15, and 20 accelerated freeze–thaw cycles over a temperature range from −45 to + 60 °C. Uniaxial compression tests, relative mass change measurements, MIP, CT with three-dimensional pore reconstruction, and SEM were employed. The results show that compressive strength gradually decreased with increasing freeze–thaw cycles, while mercury-accessible pore volume, relative mass change, and interfacial defects progressively increased. After 20 cycles, compressive strength decreased by 9.29%, while the mercury-accessible volume of large capillary pores increased by 19.05%. SEM observations of the selected fields revealed more pronounced pores and cracks near the aggregate–asphalt mastic interface. These findings suggest that the aggregate–asphalt mastic interface is one of the relatively vulnerable regions under extreme cyclic exposure, although changes in the asphalt binder itself may also contribute to material deterioration. This study provides experimental evidence for understanding the deterioration behavior of hydraulic asphalt concrete under extreme temperature conditions.