Based on indoor rainfall simulation tests carried out with a soil bin, measurements are made for various physical processes including the rainfall infiltration, the surface runoff, the sediment yield, the velocity of interrill flow and the velocity of rill flow on the purple soil slop containing different rock fragment contents. The quantitative relation between rock fragment content and soil loss is thus established. Result show that soil mixed with rock fragments can have both positive and negative influences on rainfall infiltration. The infiltration capacity of soil is enhanced when the content of rock fragments less than 20%~30%, while the capacity will be reduced after the content reaching 30%. Through different action mechanisms, soil mixed with rock fragments has different influences on velocity of interrill flow and velocity of rill flow. The increase of the rock fragment content can lead to an increase in the mean velocity of interrill flow, while decreasing that of rill flow. Through altering the composition and structure of soil, soil mixed with rock fragments can improve both soil anti-erodibility and anti-scouribility. With increasing of rock fragment content, values of sediment concentration and sediment yield will be decreased significantly. The relationship between rock fragment content and soil loss ratio can be expressed by a negative and exponential function with a high degree of reliability.