This paper applies the method of chemical dynamics to the study of soil failure and machanism of landslide for-mation, and takes Longquan purple soil as a case to study the mechanochemical behavior of the soil layer which have trend to slide under the cooperative action of the stress and water. Experiments show that, the decrease of residual shear strength of soil depends upon the pH value of solution; .the dissolution velocity of soil relates to the salt content and disso-lution curves show a S-shaped autocatalytic characteristics. which roughly confirms the possible being of autocatalytic ten-dency in mechnochemical behavior of failure of Longquan soil. Based on the experiments, the mechnochamical law of soil failure under coaction of stress and water is understood, and combination of solving residual shear strength of sliding sur-face with researching response mechanism laps the foundation for quantitative assessment of effects of underground water on destructive mechanism of soil