Based on the laboratory rainfall simulation experiments, three rill morphological indexes including rill density, rill dissection degree and rill width-depth ratio were chosen to analyze the rill morphological evolution on loessal soil slope in western Shanxi Province, and explore the effect of rill morphology on the regularity of runoff and sediment yield in different slope length during rainfall. The results showed that the rill morphology evolution process was as follow: The drop sill formed firstly, then it developed into rill and then formed rill nets, and finally the rill collapsed. The rill density and dissection degree increased with the increasing of slope length and rainfall intensity, while rill width-depth ratio decreased with the increasing of them. The impacts of slope length on rill density and rill width-depth ratio were stronger than those of rainfall intensity, while the effect of rainfall intensity on rill dissection degree was stronger than that of slope length. The relationship of average runoff rate and slope length could be described with a linear function (R2＞0.95), and the relationship of average sediment yield rate and slope length could be described with a power function (R2＞0.96). Average runoff rate and sediment yield both increased with the increasing of slope length and rainfall intensity. The correlation analysis showed that there were significant positive correlations between the rill dissection degree and the rates of runoff and sediment yield at the 0.01 level, and the correlation coefficients were greater than 0.97, and rill density was significant positive correlation with the rates of runoff and sediment yield at the 0.05 level, and correlation coefficients were greater than 0.70.