In order to study the relationship between the mineral composition and the acidity of the purple soil under the microtopography, the typical purple soil in the hilly area of Hechuan, Chongqing, was collected to simulate acidification by electrodialysis, and the mineral composition and acidity before and after the electrodialysis were analyzed by the X ray diffraction spectrum. The results showed that with the decreasing of topography, the proportion of the soil primary mineral decreased gradually, and the development degree of soil was higher and higher. The pH value of purple parent rock and its developed soils followed the order of parent rock > stony sub-soil > half sand half mud > purple yellow mud. There had a clear correlation between the change of soil pH value and the contents of calcite, albite and potassium feldspar in soil. The pH values of stony sub-soil and half sand half mud decreased 4.3 and 3.8 units respectively after electrodialysis, while the primary minerals of soil decomposed and weathered, indicating that the soil primary minerals such as feldspar minerals could buffer the acidification of purple soil.