The objective of this study was to evaluate nutrient loss under herbaceous cover in simulated field conditions and quantify the influences of the aerial and underground parts of plants in preventing nutrient loss. The experimental settings were as follows: two herbs, i.e., alfalfa (Medicago sativa L.) and green foxtail (Setariaviridis (L.) Beauv.); four covers for each herb (0, 20-30%, 50-60%, and 80-90%); and two treatments (reserving the entire plant and trimming the aerial part of the plant). Through simulated rainfall, runoff and sediment samples were collected at the end of the soil tank and analysed for nitrogen (N) and phosphorus (P). The results indicated that nutrient loss gradually decreased with the increasing plant coverage, and the nutrient loss on the green foxtail slope was lower than that on the alfalfa slope under the same coverage. Although the effect of the alfalfa canopy on nutrient loss prevention was higher than that of the green foxtail, the capacity of the green foxtail root for conserving nutrients is stronger than that of alfalfa due to a different herbaceous plant leaf area index and root morphology distribution. An exponential function was found to characterize the different plant root length densities and nutrient losses on the slope.