Abstract: As the most complicate soil erosion prediction model, WEPP model has got extensive research and application in our country. The paper ascertains the USLE and WEPP model parameters respectively and compares the measured value and predicted value, through observing the sediment yield under single rainfall in Suining group purple soil. Results showed: the most of predicted values by using WEPP model are more accurate than those by using USLE model in 20 degree fallow plot. Rainfall factor is the most important influence factors to sediment yield, no matter in fallow or measured plot, through analyzing the contributed rate of many factor. The contributed rate to fallow and measured plot produced by rainfall is 48.0% and 64.1% respectively. According to the principle that accumulating contribution rate is more than 80% in statistics, the rainfall and rainfall duration is the public factor for erosion sediment yield in Suining group purple soil.