Grass roots can improve the soil's anti-scourability. The aim of this study was to determine the effects of the roots of different grasses on anti-scourability on steep slopes of purple soil in the Three Gorges Reservoir Area. Medicago sativa, Paspalum natatu, Cynodon dactylon, and Vetiveria zizanioides plants were planted 4 years prior to the experiment, and bare control plots were selected for soil sampling. The soil anti-scourability, root parameters (root dry weight, root length, root surface area, and root volume), soil shear strength (soil internal friction angle and cohesion), and soil chemical-physical characters (bulk density and organic matter) were measured using an improved flume scouring experiment, a root analysis system (WinRHIZO (Pro.2004c)), a direct shear test experiment, the ring method, and potassium dichromate external heating method, respectively. The results show that four species of grass roots could significantly enhance soil anti-scourability. Vetiveria zizanioides roots had the greatest effect on soil anti-scourability, as soils planted with this species exhibited anti-scourability values 2.75-3.58 times that of the control; while Medicago sativa roots had the weakest effect on soil anti-scourability, as soils planted with this species exhibited anti-scourability values 1.96-2.60 times that of the control. In the scouring experiments lasting 0-10 min, the soil anti-scourability of each plot increased over time, and these dynamic change processes can be fitted well (R² = 0.9284-0.9691) using two polynomial (the control and Cynodon dactylon plots) and logarithmic equations (the Vetiveria zizanioides, Paspalum natatu, and Medicago sativa plots). Root length density and root surface area density were the main factors affecting soil anti-scourability, and both were significantly correlated with anti-scourability (P < 0.05), especially the root surface area density. Root length density and root surface area density of Vetiveria zizanioides roots were the highest among the different species. A dividing line was set at 2.00 mm, and when 0.00 mm < d ≤ 2.00 mm, all Vetiveria zizanioides root parameters were higher than those of Medicago sativa. When d > 2.00 mm, all Medicago sativa root parameters were higher than those of Vetiveria zizanioides. Roots whose diameters were 0.50 mm < d ≤ 1.50 mm increased soil anti-scourability, especially roots (1.00 mm < d ≤ 1.50 mm) whose parameters were significantly positively correlated with anti-scourability (P < 0.01). Four species of grass reduced the soil bulk density, increased the soil organic matter content, and enhanced the soil's anti-shear strength. The minimum soil bulk density and the maximum organic matter content were 1.42 g/cm³ and 13.41 g/kg, respectively, in Medicago sativa plots. The soil bulk density of grass plots was significantly lower than that of the control plots. The organic matter contents of the Medicago sativa, Cynodon dactylon, and Vetiveria zizanioides plots were significantly higher than those of the control plots, but there was no significant difference between the organic matter contents of the Paspalum natatu and control plots. The maximum soil internal friction (φ) and cohesion (c) values were 28.82° and 82.60 kPa, respectively, in the Vetiveria zizanioides plots. Soil internal friction and cohesion of each grass plot were significantly higher than those of the control plots. Soil cohesion (c) was another main factor enhancing soil anti-scourability, and was significantly positively correlated with anti-scourability. Roots (0.00 mm < d ≤ 0.50 mm and 1.00 mm < d ≤ 2.00 mm) were found to enhance soil cohesion, especially roots 1.00 mm < d ≤ 2.00 mm.