Abstract: To clarify the mechanism of nano-SiO₂（NS） modification on the strength evolution and slope stability of mudstone in an open-pit mine dump, this study systematically investigates mudstone from the Baqing Coal Mine under different NS contents （0%, 1%, 2%, 3%, 4%）. Tests include liquid-plastic limits, unconfined compressive strength （UCS）, direct shear, wet-dry cycles, and mercury intrusion porosimetry. Numerical simulations are used to analyze how the modified weak layer influences slope deformation and stability. Results show that UCS first increases and then decreases with NS content, peaking at 3% NS-approximately 4.5 times that of untreated samples. Cohesion c increases by about 98%, from 21.8 kPa to 43.2 kPa, while the internal friction angle φ rises only slightly. Under wet-dry cycles, strength decreases linearly across all samples, yet the 4% NS mix exhibits the highest strength retention（74.3%）, indicating enhanced durability. Numerical simulations reveal that the modified slope experiences about 30% less horizontal displacement, and the safety factor improves from 1.153 to 1.345, shifting the slope from “marginally stable” to “stable”. NS enhances mudstone through synergistic effects: physical filling, chemical cementation, and structural reconstruction. This study offers theoretical and engineering support for the long-term stability and eco-friendly reinforcement of mine dumps.