Abstract: This paper explores the mechanism of Nano-CaCO₃ on the mechanical properties of Guilin red clay. Triaxial shearing test under unconsolidated-undrained conditions is carried out using TSZ-1 type triaxial tester. The influence of the amount of Nano-CaCO₃ on cohesive force, internal friction angle, shear strength and stress-strain curve of remolded red clay under different dry density conditions is analyzed. The mechanism of the influence of Nano-CaCO₃ on the mechanical strength of red clay is explained from the point of view of collochemistry character of red clay mineral grain. The result of the experiment shows that with the increase of Nano-CaCO₃ content, the cohesion, internal friction angle and shear strength of red clay show a tendency to decrease first and then increase. The incorporation of Nano-CaCO₃ to change the original dielectric charge PH value changes the adsorption balance of the original iron oxide cementation to a new calcium cemented aggregate. It changes the strength characteristics of red clay. When the initial dry density is 1.4g·cm^-3, the stress-strain curve of the remolded red clay sample changes from an elastic ideal plastic strain type to a strain hardening type after adding Nano-CaCO₃.