Through the located observation trial in runoff plot in Suining Water and Soil Conservation Station, using the way of data analyzing, the laws of soil erosion on sloping farmland in Suining group purple soil. The results showsed: runoff is enlarged along with the increase of rainfall, and the relativity between the erosion and the rainfall is unapparent; the erosion thickness of sloping land, erosion amount and the loss amount of nutriment are all become big along with the increase of the cultivated land gradient;y=-30.666-1.326x_1-2.343x_2 23.646x_3 can describe the relation between soil erosion amount and rainfall, meant rainfall density, depth of muddy runoff well.