Plant roots play an important role in preventing and controlling slope instability. This study was conducted on a typical karst slope in Youyang Tujia and Miao Autonomous County, Chongqing to determine the effects of herbaceous roots on the stability of shallow layer on slopes. Ophiopogon bodinieri H. and Dolichos lablab L. were planted on the up-, mid-, and down-slopes, and bare soils corresponding to these slope positions were selected as the control (CK). Composite soil samples were collected and tested for soil properties. Root-soil composites were sampled and subjected to the direct shear tests, and then roots in these composites were picked and measured for root mass, root length, root surface area, and root volume. Slope stability factors of safety were calculated using the infinite slope stability analysis model. The key root traits affecting slope stability were determined by the structural equation model. The results showed that: (1) Ophiopogon bodinieri H. and Dolichos lablab L. improved the stability of shallow layer on slopes and the former with a fibrous root system performed better than the latter with a tap root system. Compared with CK, the slope safety factors of Ophiopogon bodinieri H. and Dolichos lablab L. increased by 149.97% to 179.96% and 52.27% to 75.53%, respectively. The maximum increment of the slope safety factors occurred at the downslope for Ophiopogon bodinieri H. and the upslope for Dolichos lablab L. (2) The roots of Ophiopogon bodinieri H. and Dolichos lablab L. increased the soil cohesion but did not significantly affect the soil internal friction angle (P ＞ 0.05). The soil cohesion of Ophiopogon bodinieri H. and Dolichos lablab L. was 1.97 to 5.11 times higher than that of CK. The root mass density of Ophiopogon bodinieri H. was 2.78 to 5.99 times higher than that of Dolichos lablab L. (3) The correlation between soil cohesion and slope safety factor was the strongest (r=0.95, P ＜ 0.01), and their relationship could be described by the positive logarithmic function (R²=0.91, P ＜ 0.01). The structural equation model showed that root mass density was the key root trait affecting the stability of shallow layer on karst slopes, with a standardized path coefficient of 0.85 (P ＜ 0.01). The optimal slope stabilization effect of herbaceous plants with a tap root system occurs on the upslope and that of herbaceous plants with a fibrous root system occurs on the downslope. Additionally, herbaceous plants with high root mass density are beneficial for slope stability. Our study can provide a theoretical basis for the scientific application of herbaceous plants on controlling karst slope instability.