To find the suitable conditions under which nano-SiO2 can exhibit a significant impermeability enhancement effect and the mechanism underlying this effect, comparisons between the permeability-related properties of a nano-SiO2-filled cement paste and those of a reference cement paste composed of different water/cement (W/C) ratios were carried out in this research. Permeability-related properties of cement paste, such as the chloride-ion penetration coefficient (D-nssm), water permeability coefficient (K-p), and initial water sorptivity coefficient (S-i), were tested. Furthermore, Power's model, mercury intrusion porosimetry data, and the general effective media theory were also applied to analyse the evolution mechanism. The results indicate that the effect of nano-SiO2 on the enhancement of the impermeability becomes more remarkable at a lower W/C ratio. The decreasing rates of D-nssm, K-p, and S (i) increase as the W/C ratio decreases. Furthermore, it can be concluded that the effects of nano-SiO2 on promoting the hydration, refining the pore structure, narrowing the width of microcrack and thus enhancing the impermeability of cement paste become much clearer as the W/C ratio decreases.