Abstract: A total of 102 samples were collected from the topsoil at a depth of 20 cm to predict and map the spatial distribution of soil pH over the Caotang River Basin in the Three Gorges Reservoir Area. The samples were divided into calibration (85%) and validation (15%) sets. Random Forest (RF) method was applied to predict the spatial distribution of soil pH based on parent materials and terrain indicators (Elevation, Slope, Aspect, Slope Height, Valley depth, Topographical wetness index, Vertical Distance to Channel Network, Multi-resolution index of valley bottom flatness, Slope Length). The major influencing environmental factors on soil pH spatial variability were investigated by the RF model. The results showed that soil pH was significantly positively correlated to Valley depth and Slope Length, while significantly negatively correlated to Elevation, Vertical Distance to Channel Network and Slope Height. Soils developed from Limestone of Triassic Daye formation had higher values of pH than that developed from Sandstone of Triassic Xujiahe Formation. The RF model had a good performance with the mean absolute error (MAE), the root mean square error (RMSE) and the determination coefficient (R²) of 0.47, 0.59 and 0.85, respectively. The model could explain 85% variation of soil pH in the study area. The major factors to soil pH variations were soil parent material and elevation. Therefore, RF model can serve as an effective method to predict the spatial distribution of soil pH, and can provide the basis and reference for other soil properties prediction at watershed scale.