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With the fast-paced growth of underground engineering, the performance demands for construction materials are increasing. It is essential to explore the performance optimization of bentonite-based materials from admixture and material property’s view. In this paper, the rheological and filtration properties were selected as the key indices of the bentonite slurry performance. The synergistic effect of bentonite particle size and content, carboxymethyl starch (CMS), and Na2CO3 content on bentonite slurry performance were investigated by conducting the rheology measurement and filtration test. Rheological behaviour, parameters, and filtration loss of the bentonite slurry with different factors were obtained. Range analysis was employed to evaluate the effect degree of each factor on the slurry performance, and an optimal bentonite-based slurry ratio was presented. Results showed that bentonite slurry exhibited shear-thinning characteristic as the bentonite content and CMS content increased, and bentonite particle size decreased. Excessive addition of Na2CO3 weakened the shear-thinning characteristic. The rheological parameters of the bentonite slurry were positively correlated with the bentonite and CMS contents and negatively correlated with the bentonite particle size. An addition of 0.5 %–1.5 % Na2CO3 resulted in the largest rheological parameter. The bentonite content had the greatest impact on the rheological parameters and filtration loss, followed by the particle size. The influences of the Na2CO3 and CMS contents were relatively weak, but the effect of the CMS content on filtration loss was stronger than that of the Na2CO3 content. An optimal ratio of bentonite-based slurry was presented for engineering reference: the bentonite particle size range of 10–23 μm, bentonite content range of 10 %–15 %, Na2CO3 content range of 0.5 %–0.8 %, and CMS content range of 0.1 %–0.8 %.