【Objective】Purple soil is a key soil resource for development of the agriculture Southwest China. A proportion of the purple soil has been somewhat acidified. A better understanding of the composition of exchangeable cations in acid purple soil and its influence on subsequent acidification processes of the soil is conducive to potential acidification risk assessment of the soil. 【Method】In this study, a total of 38 acid purple soil samples were collected from Hechuan District, Chongqing Municipality for analysis of physico-chemical properties and determination of acid buffering capacity. Acidification degree of the purple soil samples was evaluated based on their respective pH values and content of exchangeable acidity. Comparison between the samples of purple soils and some zonal soils (Alfisol, Ultisol, and Oxisol) was performed in content of exchangeable bases and cation exchangeable capacity (CEC). And then, contributions of minerals to base cations and CEC were discussed in detail. In the end, effects of exchangeable cations on acid buffering capacity of the purple soils were analyzed.【Result】Some of the soil samples, about 65%, show that the purple soil was acidified seriously with soil pH being < 5.5. Fortunately, the portion of acidified purple soil was still quite high in content of exchangeable base cations and cation exchange capacity (CEC), with Ca²⁺ being the highest in content ((13.3±5.27) cmol·kg^-1), and K⁺, Na²⁺ and Mg²⁺ being (0.267±0.190), (0.196±0.116), and (3.91±1.15) cmol·kg^-1, respectively. The average content of exchangeable base cations was 11.8 times as high as the content of exchangeable acidity, even though the highest content of soil exchangeable acidity in the purple soil reached up to 6.40 cmol·kg^-1. Compared to the acid soils developed in the subtropical and tropical regions of China, the acid purple soil was significantly higher in CEC than Untisol and Oxisol. The content of exchangeable Ca²⁺ and Mg²⁺ in the purple soil was 5.9 and 3.9 times as high as that in Ultisol. In terms of soil minerals, the purple soil was dominated with quartz, mica, K-feldspar, albite, montmorillonite, illite, kaolinite and hematite. The abundant clay minerals, especially montmorillonite, increased the amount of surface negative charge of the purple soil and, correspondingly, enhanced adsorptive ability of the purple soil for base cations. And intensified weathering of the soil replenished the soil with base cations, thus stabilizing soil pH. Further analyze of content of organic matter in the purple soil shows that the purple soil was much lower than Alfisol, Ultisol, and Oxisol in content of organic matter. As a consequence, the minerals in the soil were the main contributors of surface charges in the purple soil. Acid buffering capacity of the purple soil varied in the range of 3.18- 25.6 mmol·kg^-1·pH^-1, and was significantly and positively related to exchangeable bases and CEC in the soil, which means a high content of base cations and CEC helps enhance soil acid buffering capacity, thus retarding acidification of the purple soil.【Conclusion】Though the purple soil in some areas is quite serious in acidification, as affected by its soil forming parent materials and weathering degree of the soil, the soil has an abundance of base cations, which help retard the process of soil acidification. These are the characteristics of soil acidification of the purple soil relative to the acid soils in other areas.