Studies on the soil pH and biological mechanism of typical forest plantation in subtropical China showed that the chemical properties of soil changed noticeably after the Pinus massoniana and Schima superba were planted for 20 years in waste grassland. Compared to the waste grassland, soil pH was lower 0.12-0.47 unit in Pinus massoniana and Schima superba. In the vertical direction, soil pH at the depth of 0-20 cm under Pinus massoniana was lower than that of Schima superba, while the result was contrary at the 20-60 cm layer in soil profile at 10 cm away from the main stem. In the horizontal direction, the influence of trees on soil pH became gradually weaker with the increase of distance to the main stem. Topsoil pH value was the lowest at 20 cm apart from the main stem. A lot of H+ was input to the soil near the main stem by stemflow, which may decrease topsoil pH value. The cations uptake and redistribution by the trees were the dominant mechanism of soil acidification in the soil profile, and the difference in root distribution and cations uptake between Schima superba and Pinus massoniana were main reasons for pH change in soil profiles.