Abstract: The rainfall erosivity factor is an important parameter to quantitatively evaluate potential water erosion risk and its serious consequence in a special area. In this article, some simple algorithm of rainfall erosivity was built up by field artificial rainfall experiment in purple hilly area. The results in purple hilly area showed that: Runoff and sediment had significantly positive correlation with rainfall. Sediment was highly influenced by such period-rainfall intensity features as 5, 10, 15, 30 and 45 min rainfall intensity. Some compound factors as ∑EI5 and ∑EI10 based on rainfall kinetic energy and period-rainfall intensity could reflect the varying process of runoff in slope erosion, ∑EI5 could reflect the varying process of sediment at same time. Some compound factors as PI5, PI10, PI15 based on rainfall amount and period-rainfall intensity are the important factors affecting the varying process of runoff t and sediment in slope erosion in purple hilly area, especially PI5 is the most important factor affecting soil erosion process. The models with PI5 and ∑EI5 as dependent variables respectively could be chosen as simple algorithm of rainfall erosivity factor in purple hilly area, and the transformed relationship between these above models is, ∑EI5 = 24.16 (PI5), which can greatly improves the practical application of rainfall erosivity factor in these soil erosion predicting models such as universal soil loss equation (USLE) and revised universal soil loss equation (RUSLE). For a special rainfall event in purple hilly area, such period-rainfall intensity features as 5, 10 and 15 min are critical to reduce soil erosion and to increase exploitation of soil and water resources. Some precipitation features as 5 min rainfall intensity and its rainfall amount, raindrop distribution can take more important role for water and soil loss of slope land in purple hilly area.