[Objective] This study investigates the age-dependent evolution of macroscopic and mesoscopic mechanical properties of self-compacting concrete (SCC) used in the impermeable facing layer of rockfill concrete dams in cold and arid regions under coupled low-temperature and low-humidity curing conditions, and reveals the relationship between pore structure characteristics and mechanical properties. [Methods] Three representative coupled temperature-humidity curing regimes were established based on meteorological statistics from Northwest China during 2004-2024: standard curing (20 ℃ and 95% RH), medium-temperature and medium-humidity curing (10 ℃ and 75% RH), and low-temperature and low-humidity curing (3 ℃ and 50% RH). Uniaxial compression tests were conducted to determine macroscopic mechanical parameters, including peak stress, elastic modulus, and peak strain. Low-field nuclear magnetic resonance (NMR) was employed to quantify pore size distribution, total porosity, and fractal characteristics. Field emission scanning electron microscopy (SEM) was employed to characterize the morphology of hydration products, pore structures, and the integrity of the interfacial transition zone (ITZ). [Results] (1) Under all curing regimes, the peak stress and elastic modulus of SCC increased with curing age. However, this improvement was markedly constrained under low-temperature and low-humidity conditions, and the adverse effects on early-age mechanical properties could not be fully mitigated by extending the curing age. At 28 d, the peak stress and elastic modulus of SCC cured at 3 ℃ and 50% RH were only 60.6% and 46.3%, respectively, of those of SCC under standard curing. The peak strain of SCC decreased with increasing age in all groups, while the low-temperature and low-humidity group maintained the highest peak strain throughout the curing period (1.48% at 3 d and 1.02% at 28 d), indicating greater ductility but lower strength and stiffness. (2) Mesoscopic pore structure analysis showed that total porosity decreased with age under all curing conditions. However, the low-temperature and low-humidity group exhibited higher porosity (19.46% at 3 d and 14.41% at 28 d) and an increasing proportion of multi-harmful pores (>200 nm) from 0.15% at 3 d to 0.48% at 28 d, indicating a pronounced delay in pore structure densification. (3) Fractal analysis also showed that the fractal dimensions D₁ (harmless pores), D₂ (less harmful pores), and D₃ (harmful and multi-harmful pores) of the low-temperature and low-humidity group were significantly higher than those of the other groups at the same age, reflecting greater pore structural complexity and irregularity across different scales. The evolution of large and interconnected pores was the dominant factor controlling pore fractal characteristics. (4) SEM observations demonstrated that low-temperature and low-humidity conditions significantly delayed cement hydration. At 3 d, numerous unhydrated cement particles and loose pores were observed, while even at 28 d, the hydration products remained mainly dispersed flocculent C-S-H gel with residual pores and microcracks. In contrast, the standard curing group developed a dense and continuous C-S-H gel network with an almost indistinguishable ITZ at 28 d. [Conclusion] The findings confirm that coupled low-temperature and low-humidity curing significantly degrades the early-age mechanical properties of SCC used in the impermeable facing layer of rockfill concrete dams, and this negative effect cannot be fully compensated by prolonged curing. Mechanical property degradation results primarily from delayed cement hydration induced by the coupled environmental conditions, which leads to insufficient hydration products, persistently high porosity, continuous growth of multi-harmful pores, and markedly delayed pore structure densification. Fractal analysis further demonstrates that low-temperature and low-humidity environments increase the pore structural complexity across multiple scales, with the evolution of large and interconnected pores playing a critical role in pore structure deterioration and degradation of mechanical properties. These findings provide a theoretical basis for optimizing SCC curing strategies in cold and arid regions and highlight the necessity of targeted curing measures to mitigate the adverse effects of such environments on cement hydration and pore structure development.