Abstract: Abundant calcium-based bentonite (CaB) resources in China have garnered attentions as potential alternatives for preparing slurry when constructing cut-off walls. However, the limited swell capacity and chemical compatibility of natural CaB pose critical challenges in enhancing its engineering properties. In this study, sodium polyacrylate (PAAS) was employed as a modifier to ameliorate CaB, and optimal preparation conditions for sodium polyacrylate modified calcium-based bentonite (PAAS-CaB) were determined through the processes of slaking, drying, and grinding. Systematic testing of engineering properties and microstructure of the modified bentonite was conducted. The results reveal that at a modifier content of 8%, the swell index of PAAS-CaB reached 34.0 mL/2g. Sodium polyacrylate modification significantly enhanced the water absorption and swell performance of CaB, along with improving its chemical compatibility, resulting in reduced permeability coefficient of the bentonite cake. With increasing PAAS content, the marsh viscosity of slurry increased, while filtrate loss and pH value decreased. Slurry density was primarily controlled by the bentonite content. Compared to the unmodified CaB, PAAS-CaB exhibited a superior overall lamellar structure at microscopic scale, characterized by a smaller interlayer distance of montmorillonite crystalline layers. Sodium polyacrylate primarily modified calcium-based bentonite through bridging encapsulation and graft copolymerization. This study establishes a reliable foundation for optimizing the preparation process of PAAS-CaB and presents a feasible technical pathway for the engineering application of calcium-based bentonite.