The soil erosion is little concerned in low mountains and hills of south Jiangsu province than in the loess plateau and the red soil area because the absolute erosion is small. However the ecological impact caused by soil erosion in south Jiangsu province can not be ignored since the nutrient loss and the soil layer becoming thin there is greater than that in the loess soil and the red soil area. In order to discuss the ecological impact of the soil erosion there the soil erosion rate should be known. The tracer method of cesium 137 ( 137 Cs) is a useful tool. But the two problems should be solved for using this method. One is to decide the 137 Cs reference inventory in the study area and the other is to set up the models of estimating soil erosion. After careful study the 137 Cs reference inventory is decided as 2 200 Bq/m 2 in our study area. The soil erosion estimating models of the cultivated soil and the uncultivated soil are build based on the fundamental physical content. The soil erosion rate and the soil sedimentation rate of the hillside fields in bamboo forest, in tea plantation and so on can be estimated using above two soil erosion models and a soil sedimentation model provided by the other scholars. Then the ecological impacts caused by soil layer becoming thin and nutrient loss are tried to analyze.