【Objective】 Purple soil is extensively distributed in Southwest China. To study acidification characteristics of the purple soil and its parent purple rock in the hilly areas of that region as affected by micro-topography, this study was launched. 【Method】In this study samples of two kinds of purple rocks and four types of purple soils were collected from the hilly area of Southwestern China for analysis of physicochemical properties in the lab. Samples of the two kinds of purple rocks, mudstone and sandstone, were collected at the top of hills and of the four types of purple soils, stony sub-soil, half sand half mud, bigeye mud and purple yellow mud, distributed in sequence along hill slopes from the top to the bottom, were done, respectively, too, 14 samples each. 【Result】 Acidification, to a varying extent, was found in the purple soils of that region. Consequently, in these soils the content of exchangeable acidity increased, and the content of total exchangeable base cation and the base saturation decreased. Purple soil development gradually increased in degree along hill slopes from the top to the bottom. The carbonate in the soils that helped keep soil pH in the range of 8 to 12 was subjected to leaching and the impact increased too along the slope from the top to the bottom. As a result, soil acidification was gradually extending in range along the slope. In terms of acidification degree, the four soils followed an order of purple yellow mud > bigeye mud > half sand half mud > tony sub-soil. In acidified purple soils, pH was controlled by cation exchange reaction, and affected by the degree of base cation (K⁺ , Na⁺ , Ca²⁺ and Mg²⁺ ) subjection to leaching and recovery, which varied with micro-topography. The stony sub-soils on the top of hills were exposed to leaching of base cations only, while the bigeye muds and half sand half muds on the mid slope were subjected to supplementation of base cations leached from the stony sub-soils on the top of the slope, besides leaching loss of base cations. The purple yellow muds, at the bottom of the hill or slope, highly weathered, were low in effective cation exchange capacity and hence the colloids in the soil adsorbed less H+ and Al3+ on the surface. Generally speaking, in terms of overall acidification degree, the four types of purple soils followed an order of stony sub-soil > half sand-half mud > bigeye mud > purple yellow mud. 【Conclusion】It can be concluded that as affected by micro-topography, acidification of the four types of soils increases gradually in range from the top to the bottom of the hill, but does not in degree. Therefore, it is essential to pay attention to the acidification characteristics of the purple soils of Southwest China in exploiting the soils.