<p>Conventional geotechnical approaches often overlook the evolution of micro-particle-scale interface mechanics and their influence on soil strength. This study employed a non-destructive sampling method combined with nanoindentation test to investigate the microparticle-scale interface mechanical properties of arsenic (As)- and cadmium (Cd)-contaminated soils. Complementary molecular-scale analyses including Fourier transform infrared spectroscopy (FTIR) and zeta potential measurements, together with micro-particle-scale laser diffraction particle size analysis, revealed the evolution mechanism of micromechanical properties. The results showed that the tight chemical coordination or adsorption between Cd and mineral surfaces compressed the diffuse double layer (DDL) and induced particle aggregation, which strengthened micromechanical nanoindentation properties. Conversely, as increased the negative surface charge, promoted particle dispersion and weakened micromechanical strength. When presented together, As and Cd exerted antagonistic effects, resulting in intermediate multi-scale responses. Correlations with direct shear testing showed that changes in microscale indentation modulus (<i>E</i>) and hardness (<i>H</i>) closely tracked shifts in macroscale shear strength, especially soil cohesion (<i>c</i>). These findings highlight how contamination-driven variations in DDL interactions govern microscale interface mechanics and overall engineering performance. This multi-scale framework emphasizes the necessity of incorporating micro-scale interface mechanical properties in geotechnical practice, particularly in contaminated environments.</p>