The Chinese Grain for Green Project is crucial for controlling water and soil erosion, rehabilitation forest vegetation, and improvement of the environment. Soil acidification could be induced during the vegetation restoration, which has a negative impact on plant growth. It is important to clarify the degrees and mechanisms of the influences of reforestation species on soil acidification in order to achieve a better ecological benefit of the Grain for Green Project. In this study, we used the Purple soil of sloping farmland (planted Zea mays) as control group (SF) in rainy area of western China, and analyzed the changes and profile distribution of soil pH value after a 20-year farmland to artificial forestland with plant species of Cryptomeria fortune (CF), Cryptomeria fortune & Betula luminifera (MF), Neosinocalamus affinis (NA) and Camellia sinensis (CS). Results showed that the weighted average value of soil pH in the top 50 cm was 5.66 for SF, 5.55 for NA, 5.12 for CS, 5.03 for MF, and 5.00 for CF. Soil pH value of CF, MF and CS decreased significantly compared with SF (P<0.01), and these soils exhibited severe acidification. In addition, vegetation restoration increased the degree of vertical variations of soil pH for SF that showed a uniform profile distribution. By contrast, soil pH for artificial forests exhibited greater profile differentiations. Soil pH values (4.89-5.90) of the 10-50 cm layer for different artificial forests were significantly higher (P<0.01) than those (4.72-5.21) of the 0-10 cm layer suggesting that soil acidification was most intense for the top 10 cm. There were significantly negative correlations between soil pH value and soil organic matter content, while significantly positive correlation between soil pH value and Na/K ratio of weathering index was observed. 53.9% of the variation for soil pH was explained by soil organic matter content and Na/K ratio, indicating that the acidification of Purple soil in this area were driven by the accumulation of soil organic matter during the process of returning farmland to forests and sorts of feldspar weathering. By revealing the acidification characteristics and its processes for Purple soil, this study provides a scientific basis for future vegetation restoration and ecosystem management in rainy area of western China.