Grass hedgerows are beneficial for preventing soil erosion on karst slopes, and their roots play a vital role. This paper aimed to analyze the morphological and mechanical properties of the root system of different hedgerows, to quantify the shear strength and anti-scourability of the root-soil complex, to explore the evaluation factors of the soil erosion resistance of the hedgerows in karst area and to provide reference for the scientific application of plant restoration measures in soil erosion control on karst slopes. Dolichos lablab L. and Vicia villosa hedgerows were planted on the upper, middle and down slopes of karst bedding slope in Youyang, Chongqing. The physical properties, root morphology and fiber content, tensile properties, shear strength index and anti-scouring index of the root-soil complex were measured. (1) The mean root length density and root surface area density of Dolichos lablab L. were 59.32% and 16.86% higher than those of Vicia villosa in the shear complex. The mean root length density, root surface area density and root volume density of Dolichos lablab L. were 30.48%, 57.78% and 92.98% higher than those of Vicia villosa in the souring complex. The ultimate tensile force and tensile strength of Dolichos lablab L. root were significantly higher than those of the Vicia villosa. (2) The roots of the two kinds of grass hedgerows could enhance the shear strength and anti-scourability of soil. Compared with the control check, the cohesion of Dolichos lablab L. and Vicia villosa complex was 113.06%-124.37% and 51.56%-87.12%, respectively, and the anti-scouring index was 2.81 and 2.45 times higher than the control check. (3) In different slope positions, the root length density was significantly higher in the lower slope than in the upper and middle slope. The tensile strength of Dolichos lablab L. root was the best in the downslope and the Vicia villosa was the best in the upslope. The cohesion and anti-scouring index of Dolichos lablab L. complex were both downslope > middle slope > upslope. (4) The principal component analysis showed that the comprehensive scores of the shear strength and anti-scourability of the root-soil complex of the two kinds of grass hedgerows were the highest in the down slope. Root surface area density, root tensile strength and root lignin content had the highest contribution to the shear strength and anti-scourability performance of the root-soil complex, which could be used as important parameters to characterize the shear strength and anti-scourability of the root-soil complex. When grass hedgerow measures are implemented in karst areas, planting on the low slope should be carried out in order to give full play to the soil and water conservation effect.