Rill erosion is one of the main form of slope erosion, which is between sheet erosion and gully erosion. Once rill erosion takes place, the rate of soil erosion will be double increased. It is important, through Investigating the critical slope length of rills occurred in different slope cultivated lands under torrential rain, to direct the prevention of rill, reduce sediment and preserve the water and soil. Utilizing artificial rainfall tests showed that, under the same soil moisture content and in a 130mm /h torrential rain, the mean critical slope length on purple cultivated land of slope angle 10 °， 15 °， 20 °， 25 ° were 6.13 m ， 4.12 m ， 2.72 m ， 1.55 m ， respectively. A quatratic parabola relationship between critical slope length of rill ( L_R) and slope grade (J) was given. When the water content of soil varied from 16% to 26%, the critical length increased firstly and then decreased. For the sake of avoiding rill erosion occurrence, we designed a new cultivated mode, named “ ridge & plant hedge ”， which was to cut the slope length by ridges and hedgerows,on one hand to decrease sediment, on the other hand to regulate water content of soil.