Granite residual soils are highly sensitive to water, leading to softening and disintegration that poses serious risks to engineering geosafety. Traditional soil improvement methods focus on structural modifications, overlooking the crucial role of surface properties in hydraulic transport. This study introduces Octadecylamine (ODA) as a surface modifier, which alters the surface contact angle of the soil without affecting its internal structure. We explored the relationship between water ingress/egress capacity and contact angle using various tests (contact angle, water droplet infiltration, SWCC, water loss, SEM, disintegration). Results show that ODA increases the contact angle as water and ODA content rise, transitioning the soil from hydrophilic to hydrophobic. The soil-water characteristic curve analysis indicates that hydrophobicity reduces the soil’s water-holding capacity and decreases the hysteresis loop area. Additionally, the water loss and disintegration tests reveal enhanced evaporation and disintegration rates with increased contact angle, improving water stability. SEM analysis shows that the soil’s pore structure remains unchanged, confirming that the performance improvement is due to surface property changes, not structural alterations. This study presents a novel surface modification technique for mitigating the risks of granite residual soil, offering valuable insights for engineering protection.