The geological and geomorphic environment is complex in karst areas in southwest China, which has seriously hampered the study of soil erosion in these areas. For this reason, the 137Cs tracer method, remote sensing, and Geographic Information System (GIS) technology based on high-precision digital elevation model (DEM) data are used to estimate the soil erosion rate from a point to the surface and for the comprehensive evaluation of the spatial distribution and erosion mechanism of soil erosion on a karst slope. The average 137Cs areal activity of nine sampling sites reached 875 Bq/m2 (background value of 960 Bq/m2), the uncultivated land profile of 137Cs was obtained in the topsoil layer of 0–6 cm, and the cultivated land profile of 137Cs was obtained in the topsoil layer of 0–20 cm. The soil erosion intensity in the investigation area was dominated by micro-degree and moderate erosion, and the average soil erosion modulus of uncultivated land was higher than that of cultivated land. The intensity of soil erosion at the slope top was lowest and increased continuously along the slope direction on a hillslope with a short slope and a steep gradient, whereas for a hillslope with a long slope and a low gradient, soil was easily deposited at the slope toe. The area ratio of the slope gradient range of 17–23° was 31.9%, the erosion ratio was 32.2%, and the deposition ratio was 32.9%. On the uncultivated land, the erosion prediction results of the geo-spatial interface for the Water Erosion Prediction Project (GeoWEPP) model were consistent with the estimation results of the 137Cs moving boundary model, but on the cultivated land, the erosion prediction results of the former were larger than those of the latter. Overall, the methodology combining 137Cs and GeoWEPP is favorable for monitoring and assessing regional soil erosion in uncultivated land of a moderately rocky desertification area, but for cultivated land, GeoWEPP gives relatively poor results and must be improved and optimized.