In response to the mechanical characteristics of contaminated expansive soil, unconfined compressive strength and direct shear tests were conducted to explore the changes in mechanical characteristics at the macroscopic level, combining SEM experiments to analyze the changes in microstructure. After XRD experiments, JADE analysis software was used to quantitatively analyze the collected raw data and obtain the trend of changes in the proportion of each phase content in the sample, in order to explore the mecha-nism of changes in the mechanical properties of expansive soil. The experimental results show that under the same compaction degree and Mn²⁺ concentration, the expansive soil sample with the optimal moisture content has the highest strength; Under the same initial moisture content, the strength, internal friction angle, and cohesion of the sample all show an increasing trend with the increase of Mn²⁺ concentration; Quantitative analysis of the phase by JADE indicates that SiO₂ in the soil shows an increasing trend after being contaminated with Mn²⁺. The reason is that the hydrolysis of Mn²⁺ in MnSO₄ solution indirectly leads to the hydrolysis of silicate minerals in the expansive soil sample, releasing SiO₂ and increasing the bonding effect between soil particles, thereby improving the overall integrity and stability of the soil.