In response to the decrease in slurry viscosity and the occurrence of segregation and bleeding caused by seawater intrusion during slurry shield tunneling in marine strata, the effects of different preparation methods, including various expansion methods and Na2CO3 modification, on the properties of bentonite slurry mixed with seawater were investigated. Parameters such as viscosity, bleeding rate, particle size distribution, and Zeta potential were measured to analyze the influence patterns and to explore the mechanisms behind slurry property changes. The results indicate that the expansion method significantly affects slurry properties. By using freshwater expansion, bentonite hydration and expansion were effectively promoted. Compared with seawater expansion, the bleeding rate was reduced by approximately 30%, a slightly higher absolute Zeta potential was observed, and the smallest particle size was obtained, resulting in overall superior performance relative to seawater-swollen and mixed-swollen slurries. No significant influence of Na2CO3 addition on slurry viscosity was observed, whereas the bleeding rate was markedly reduced. The influence of the expansion method on slurry performance was primarily governed by cations. Na? in freshwater facilitates the formation of a thicker double electric layer, promoting bentonite hydration and dispersion and for a stable slurry system. In contrast, polyvalent cations such as Ca2+ and Mg2+ in seawater compress the double electric layer, inhibit dispersion, and thus deteriorate slurry performance. Na2CO3 improves slurry performance through the precipitation of polyvalent cations and Na+ exchange, which increases the Zeta potential and expands the double electric layer. This study proposes an integrated regulation strategy combining freshwater pre-expansion with Na2CO3-based chemical salt resistance for bentonite slurry used in slurry shield tunnelling in marine formations, providing a reference for bentonite type selection and slurry preparation process optimization under high-salinity conditions.