[Objective] Plant roots play a key role in controlling soil erosion. Under different planting density the effects of roots on soil consolidation and erosion reduction should been explored. [Methods] The typical herbaceous plants-ryegrass (Lolium perenne) and alfalfa (Medicago sativa) on the Loess Plateau were selected as the research object. By setting 100, 300 and 500 plants m-2, the effect of plant root growth on soil resistance to erosion was studied experimentally.[Results] During the observation period, the root characteristics showed a trend of rapid increase at first and then slow decrease. Those of ryegrass and alfalfa were the best in medium and high planting density plots, respectively. Driven by root growth, soil organic matter showed an increasing trend. That of ryegrass and alfalfa increased the most in medium and high density planting plots, which increased by 2.44 g/kg and 1.84 g/kg, respectively. Under the combined influence of changes in root and soil characteristics, riil erodibility index decreased rapidly at first and then showed a fluctuating change during the observation period, while soil critical shear stress did not show a stable trend. In addition, the ryegrass roots had the best erosion reduction effect in the medium density planting plot, where rill erodibility index decreased by 18.86 s/m. The alfalfa roots had the better erosion reduction effect in the medium and high density planting plots, where rill erodibility index decreased by 13.42 s/m和10.82 s/m, respectively...[Conclusion] Considering the effect of planting density on root and soil characteristics, the medium density (300 plants m-2) and high density (500 plants m-2) planting patterns in this study were suitable for soil erosion control of ryegrass and alfalfa, respectively.