Abstract: The abundant calcium-based bentonite (CaB) resources in China have gained attention as potential alternatives for preparing slurry when constructing cut-off walls. However, the limited swelling capacity and chemical compatibility of natural CaB pose critical challenges to enhancing its engineering properties. In this study, sodium polyacrylate (PAAS) is employed as a modifier to ameliorate CaB, and the optimal preparation conditions for sodium polyacrylate-modified calcium-based bentonite (PAAS―CaB) are determined through the processes of slaking, drying and grinding. The systematic tests on the engineering properties and microstructure of the modified bentonite were conducted. The results reveal that at a modifier content of 8%, the swelling index of PAAS―CaB reaches 34.0 mL/2g. The sodium polyacrylate modification significantly enhances the water absorption and swelling performance of CaB, along with improving its chemical compatibility, resulting in reduced permeability coefficient of the bentonite cake. With the increasing PAAS content, the Marsh viscosity of slurry increases, while the filtration loss and pH value decrease. The slurry density is primarily controlled by the bentonite content. Compared to the unmodified CaB, PAAS―CaB exhibits a superior overall lamellar structure at a microscopic scale, characterized by a smaller interlayer distance of montmorillonite crystalline layers. The sodium polyacrylate primarily modifies the 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 the calcium-based bentonite.