Assessment of the variation of soil erosion and sediment mobilization at different hillslope positions using the 137Cs tracing technique has been carried out for the Dapotou closed watershed, a representative depression in the karst gabin basin in Southwest China. The results showed that the annual soil erosion rates in the shoulders, backslopes, and footslopes were 0.87, 0.35 and 0.49 cm year−1, respectively, while the soil sediment deposition rate in the depression bottom was 2.68 cm year−1. The average annual soil erosion modulus of the complete hillslope was 632 t km−2year−1, which confirmed the seriousness of erosion according to the gradation of the karst soil erosion standards. For the whole catchment, the sediment delivery ratio was estimated as 0.82. To identify which factor could play the most important role in influencing the estimates using 137Cs, a linear correlation and principal component analysis were conducted. The results showed that 137Cs concentrations at different soil depths of the different hillslope positions were significantly correlated with soil organic matter and total nitrogen (P < 0.05). As this watershed is a typical karst geomorphological type, these findings are expected to provide data support for larger watershed soil erosion management and ecological restoration in fragile karst ecosystems.