Surface roughness of farmland, as a result of the joint effects of land management and soil erosion, has a critical impact on the processes of surface runoff and soil erosion. Through simulated rainfall experiments, investigation was made of effect of soil surface roughness on rainfall erosion as affected by slope position on a loess slope, using high-precision 3-D laser scanning techniques. Results show that of the same slope, the top, side and foot was significantly different in statistical characteristics of elevation and the effect of soil surface roughness on rainfall erosion was significantly affected by slope position. Soil surface roughness was found to be in significant linear relationship to gradients of micro-slopes. It is, therefore, concluded that the effect of surface roughness on rainfall erosion varies spatially.