Abstract: To address the softening of soil and slope instability caused by rainwater erosion at earthen sites, Nano-SiO₂ and sodium carboxymethyl cellulose (CMC) composites were utilized to enhance the impermeability and erosion resistance of the soil. Using the Zhouqiao site soil in Kaifeng City as the research object, composites with various mixing ratios were prepared and tested for water stability, shear strength, and permeability after drying. The results indicated that increasing the composite mixing ratio significantly improved the water stability and shear strength of the soil while reducing its permeability coefficient. The optimal ratio was found to be 0.75% Nano-SiO₂ and 0.6% CMC, which resulted in the least disintegration of the modified soil. Compared to the unmodified soil, the cohesive force and internal friction angle increased by 19.87 kPa and 5.5°, respectively, while the permeability coefficient decreased by 10.47×10⁻⁷cm·s⁻¹. Rain erosion resistance tests revealed that under a rain intensity of 50 mm/h, gully erosion on the unmodified slope began after 16 minutes, with an erosion area of 0.15 m², accounting for approximately 50% of the total erosion area. The runoff strength of the unmodified soil was 0.252 kg/min. In contrast, the erosion characteristics of the modified slope were negligible, with the runoff strength reduced to 0.071 kg/min, demonstrating the excellent water and erosion resistance of the modified soil. These findings highlight the potential of the composite material for improving the water stability and erosion resistance of site soil, thereby reducing soil erosion and enhancing slope stabilization. This has significant implications for the protection of earth sites and slope stability.