In this paper, the mechanism of heavy metal ions in soil pores affecting the physical-mechanical properties of soil is studied by laboratory experiments. In order to figure out the change laws of heavy metal ion on the Atterberg limit and undrained shear strength, different concentrations of heavy metals Cu_²⁺,Zn_²⁺,Pb_²⁺were mixed with kaolinite, illite clay and sodium bentonite. The experimental results show that the liquid limit and plasticity index of low active soil with kaolinite and illite as the main clay minerals increase with the increase of ion concentration, while sodium bentonite shows the opposite change. The undrained shear strength of heavy-metal-contaminated soils is similar to the change of liquid limit. Based on the existing quantitative correlation between physical and mechanical properties of non-polluted soils, the quantitative relationship between undrained shear strength and liquidity index of heavy metal contaminated soil was analyzed. It was found that the change of undrained shear strength caused by heavy metal ions can be attributed to heavy metal ions. The resulting changes in liquid limit and plastic limit indicate that there is almost no chemical reaction in the process of heavy metal pollution, and the physical-mechanical quantitative relationship is consistent with the existing non-polluted soil empirical relationship.