Abstract: Taking GuanLing-Xiangle Small Watershed as the research area, the relationship between the occurrence of surface erosion and the early effective rainfall in the Karst area was analyzed by the fractal distribution theory to calculate the effective prophase rainfall of the basin. Results showed that both the accumulative frequency of the erosion and sediment yield and rainfall, and the accumulation rainfall threshold of the sediment and rainfall time followed the fractal power exponent relation. The early effective rainfall model can be established by the fractal distribution theory to combine the rainfall and sediment yield in the Karst basin, and the calculation results by this model was more practical than the traditional model. Combining the daily rainfall with the effective early rainfall determined the rainfall threshold of the erosive rainfall and sediment yield in the Karst river basin and was of great significance for the prediction of erosion and sediment yield in the Karst valley.