We have revised some of the parameters of the classical USLE model,through 3S technology,and conducted a numerical simulation and quantitative analysis of soil loss in the Longchuangjiang River watershed in the upper reaches of the Yangtze River valley.The main soil loss model was calculated as,Q=Cr.Re.Ke.LG.VcP.and the predicted intensity of soil erosion was estimated by the model,Sp =VcP-Vc P .Some parameters,such as Re-value,Ke-value,Pc-value,LG-value and RE-value,were revised according to monthly rain fall data of one representative hydrological station and nine ordinary hydrological stations during the flood season.The ke-value was revised by investigating the soil characterstics of 111 soil profiles,and the determination of Vc-value types were revised using the distinguished rate ETM images and calculating the values of slope degree and slope length (LG) using imagery pixel methodology.The simulated results were as follows:1) The amount of soil erosion in 1998 (drought year) was 840069.19 t,the erosion area was 2293.7841 km2,the amount of soil erosion in 1999 (water-logged year) was 8690182.6 t and the erosion area was 2293.7841 km2.The A value of 1998 was 55.7% of 1999.Severely eroded areas in 1998 and 1999 occupied 66.80% and 85.20%,respectively,of the total valley area; 2) Moderately eroded areas were 35.2% of the total area in 1999 and 26.0% in 1998; 3) Areas in immediate need of soil erosion control and management occupied 16.27% in 1999 and 14.63% in 1998 of the entire watershed; and 4) we compared the simulated value with observed values.In 1999,the simulated A-value was 8690182.6 t,but the observed value at Xiaohekou hydrological station was 991000 t,only 1/8 of the simulated value.Because the amount of sedimentation at Daguoba,Xiakouba,Chushuang,Lanniqing,Maobanqiao,Jiulong Dian,Fengtun and Qingfeng stations have not been measured,the amount of silt loss monitored at Xiaohekou station did not represent the amount of silt loss for the whole valley (research area).Quantitative monitoring and numerical prediction of the mosoon-red soil region of Southwest China using these methodologies produced rapid and practical results.However,comparison between numerical simulations and monitoring at hydrological stations will be further investigated in the future.