Abstract: To enhance the service performance of concrete in cold regions under sub-zero temperatures, this study systematically investigates the improvement effect and mechanism of nano-alumina (NA) on the macroscopic mechanical properties and microstructure of concrete cured at −5℃. By employing macro- and micro-scale testing methods such as compressive strength tests, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM), the influence of different NA dosages (1%, 2%, and 3%) on the performance of concrete under sub-zero curing was analyzed. The results indicate that sub-zero curing leads to a significant decrease in the compressive strength of ordinary concrete (OPC), with a 28-day strength reduction of up to 40%. The incorporation of 1% NA effectively mitigates this strength loss, restoring its compressive strength to approximately 75% of that of the reference group under standard curing conditions, demonstrating better performance compared to other dosage groups. Microscopic analysis reveals that NA can still exert its nucleation effect and physical filling role even in a sub-zero environment, promoting cement hydration, optimizing the pore structure, significantly reducing the most probable pore size and critical pore size, decreasing the proportion of more harmful pores, and thereby enhancing matrix density and structural integrity. This study demonstrates that NA retains significant modification potential under sub-zero conditions, with an optimal dosage of approximately 1% for achieving the best improvement effect. The research findings provide key theoretical support and technical references for the practical application of nanomaterials in concrete engineering in cold regions.