Abstract: During slurry shield tunnel construction, tunnel face stability relies on the formation of a low-permeability filter cake using bentonite slurry. However, soil intrusion during excavation alters slurry properties, posing challenges to tunnel safety and excavation efficiency. To systematically investigate the influence of fine sand on slurry rheology and filter cake formation, a series of rheological tests and modified fluid loss tests were conducted. The experimental design involved preparing slurries with 40 g·L^-1, 50 g·L^-1, and 60 g·L^-1 bentonite, supplemented with fine sand to achieve low (1.05 g·cm^-3), medium (1.10 g·cm^-3, 1.20 g·cm^-3), and high (1.30 g·cm^-3) densities. Bentonite concentration primarily controls yield stress and plastic viscosity. Sand disrupts the clay gel network, reducing yield stress. Plastic viscosity initially decreases and then increases with sand content. Higher bentonite concentrations reduce filtrate loss by forming dense, low-permeability filter cakes. In contrast, sand creates porous cakes through particle bridging, increasing filtrate loss. Notably, all filter cakes maintain ultra-low permeability (10^-9 m·s^-1), ensuring pressure sealing integrity. Additionally, sand-modified rheology reduces hydraulic gradients in pipeline flow. This work clarifies how fine sand mixing affects slurry rheology and filter cake formation, enabling slurry optimization for supporting both tunnel face stability and excavation efficiency in sand-rich ground.