<p>The significant stockpiles of Shield Tunneling soil (STS) generated by Large-scale subway construction can be recycled as road material, a practice which alleviates environment pressure but also reduces the cost. However, the residual Sodium Polyacrylate dispersant (PAANa-D) in STS significantly affects its Atterberg limit, including plastic limit (PL) and liquid limit (LL), resulting in a substantial variation in the mechanical property. Moreover, the considerable fluctuation in PAANa-D concentration adds significant uncertainty to this impact. To promote the recycling of STS, this study aims to investigate the impact mechanism of PAANa-D at different concentrations through a suite of test on three main clay minerals, including Ca-Montmorillonite, Kaolinite and Illite. To analyze the impact mechanism from the perspective of the bound water and PAANa-D function, the nuclear magnetic resonance test was employed to measure the thickness of strongly bound water film. The sedimentation test was employed to evaluate the transformation of function. The zeta potential test was employed to analyze the transformed mechanism from the perspective of the interaction between PAANa-D and particles. Meanwhile, the transmission electron microscope and rheology test was employed to analyze the mechanism from the perspective of the interaction of molecules. The results indicate that with the increasing concentration of PAANa-D, the PL and LL of STS decreased first and then increased. This impact also extended to the optimum moisture content and maximum dry density. These variations were primarily attributed to the function of PAANa-D rather than the thickness of strongly bound water. At the lower concentration, the Zeta potential decreased with the increasing concentration. The adsorption of PAANa-D enhanced the dispersion of particles. At the higher concentration, the Zeta potential increased with the increasing concentration. A large number of Na⁺ results in a loss of PAANa-D dispersion function. In addition, the molecular conformation evolved from rod to coil, and the chains originally dispersed in solution entangled with each other, resulting in the formation of the molecular cluster. Under this condition, the PAANa-D molecules bridged the particles, and its function transformed from dispersing to agglomerating. These findings provide a systematic insight into the relationship between polymer dispersant and Atterberg limit. They also offer a theoretical foundation for addressing key challenges in the recycling of STS.</p>