The algorithm of slope length factor based on up-slop runoff area was mended and new algorithm of slope length factor considering the effect of land use/cover for up-slop runoff was produced. By using geographic information system (GIS) and the revised universal soil loss equation (RUSLE), the soil erosion intensity and its relationship with environmental factors in the Simianyaogou watershed, located at Loess Plateau were studied. The results showed that the average annual soil erosion intensity in the watershed was 4 399.79 t/(km2·a), which was in the category of moderate degree erosion. Both soil erosion intensity and amount increased significantly with the increasing of the slop gradient. 80.59% of the total soil loss occurred in the region with a gradient more than 25 degree, of which the area was 59.06% of the total watershed area. Soil erosion intensity varied with slope aspects in a trend of sunny slope＞half-sunny slope＞half-shady slope＞shady slope. The area of sunny slope occupyed 45.07% of the total watershed area, but the erosion amount of which occupyed 56.50% of the total erosion amount. In different land use types, the grassland occupyed 57.07% of the total watershede area, but the erosion amount of which occupyed 96.37% of the total erosion amount. So, grassland had became the major erosion and sediment source in the watershed. The study provides technical basis for applying RUSLE to assess soil erosion on Loess Plateau and offers useful references for water and soil resource utilization in the region.