To explore the effect of Dolichos lablab L. hedgerows on soil and water conservation, we planted Dolichos lablab L. at up, middle, and down slopes at bedding slopes of the karst valley in Youyang Chongqing. The roots and root-soil matrix were sampled. The mechanical properties of roots, soil anti-scourability, and soil shear strength of root-soil matrix were tested. To find the factors affecting mechanical properties, we scanned root grayscale and tested root chemical compositions (lignin, cellulose, and hemicellulose), and also performed correlation analysis. The results showed that:(1) The whole root parameters (root length, root surface area, root volume, and root weight) had the highest value in the middle slope portion, followed by down slope portion; the corresponding value of the up slope portion was minimum. Root dominant diameter was in the range of 0.0<d≤1.0 mm. The average content of cellulose in plant root was higher than that of lignin and hemicellulose. The root cellulose content was significantly negatively correlated with root diameter. (2) The root average ultimate tensile force of Dolichos lablab L. was 6.77 N and the average tensile strength was 29.57 MPa. Cellulose and lignin had statistically significant influences on the root mechanical properties. Their regression relations are as follows:the ultimate tensile force increased logarithmically with lignin content and increased linearly with lignin/cellulose(wood fiber ratio); tensile strength increased logarithmically with cellulose content, decreased logarithmically with wood to fiber ratio. (3) The root volume density, root weight density, and wood fiber ratio of the up slope shear strength samples were significantly greater than those of the middle and down slopes. However, for the anti-scour samples, root volume density, root weight density, cellulose content, and lignin content were higher at middle slope portions but there was no significant difference. (4) The root system could effectively improve the shear performance of the soil. The internal friction angle and cohesion of the root-soil matrix were higher than those of the bare ground control. Significant differences in cohesion was only observed for the up slope portion, between the terrace hedgerow and bare ground control, increasing by 45.67%. The internal friction angle and anti-scourability were better at middle and down slopes. Our study showed that the mechanical properties of the root-soil matrix were mainly affected by root volume density, root weight density, root surface area density, and cellulose content. In summary, the root diameter in the range 0.0<d≤ 1.0 mm substantially contributed to the mechanical properties of Dolichos lablab L. root and root-soil matrix. The anti-scourability of root-soil matrix was better at the middle and down slope portions; the shear performance of root-soil matrix was the best at the up slope portion, but anti-scourability was the worst. When promoting the planting of terrace hedgerows of Dolichos lablab L. soil and water conservation would be improved when they are planted in low and flat terrains (such as the middle and down slope portions).