Studies of relief amplitude are of significance and practical application values not only to the sensitivity analysis of geological hazards like landslide and debris, but also to evaluation of the risk of geological hazards. However, little is known about relief amplitude in medium and small scale watersheds. Our objectives were to characterize the best relief amplitude of optimum statistical unit area of medium and small scale watersheds, learn geomorphic features of watersheds of such scales and figure out the reason of geological hazards. We chose the Wangmo River Watershed located in southwestern area of Guizhou Province and famous for its frequent landslides as our study area. We used a geographic information system (ArcGIS) and the DEM of 1:10000 scale to compute the optimum statistical relief amplitude unit of the watershed. Firstly, we adopted window analysis method and python module programming based on ArcGIS to automatically extract the different areas corresponding to the relief; condly, we employed mean change point analysis method and python module programming to automatically calculate the optimum statistical relief amplitude unit of the watershed.Finally, we analyzed the relief amplitude characteristics of the watershed and the relationships between relief amplitude, average slope and stream gradient factors. The results revealed that: 1) There was a significant logarithmic relationship between relief amplitude and average cell area of the watershed. 2)Based on python module programming we can automatically calculate the statistically optimum relief amplitude unit, that is 1.49 km², and the relief amplitude ranged between  0~648.23 m. 3) The reliefamplitude is higher in the middle of the areas where river networks are concentrated, while it is lower at the edge of the plateau, and the rivers are mainly distributed in the range of 200~500 m of the terrain. The landscape belongs to small relief mountain. 4) There is a significantly positive correlation between stream gradient and relief amplitude, that is, the stream gradient increased with the increase of the relief amplitude. These results show that the Wangmo River Watershed belongs to small mountain geomorphology, with cutting highlands and cutting mountains as well as serious soil erosion.