To study the effects of herbs’ root systems on soil shear strength in a collapsing gully alluvial fan. Pennisetum sp. and Paspalum wettsteinii were chosen as the research objects. A root analysis system (WinRHIZO) was used to characterize the root systems, and the in-situ shearing test was applied to analyze soil shear strength. The results showed that: (1) Root systems could improve soil structure and water status. (2) Root systems of both herbal plants were mainly distributed in the 0-5 cm soil layer, in which root parameters of Pennisetum sp. and Paspalum wettsteinii were 3.1 to 4.39 times and 2.23 to 2.57 times as great as their average values in the whole soil profile, respectively. Root parameters of Pennisetum sp. and Paspalum wettsteinii showed a logarithmic and linear decrease with the increase of soil depth, respectively. (3) The average soil shear strength followed the order of Pennisetum sp. > Paspalum wettsteinii > bare land, being 21.04, 16.43, and 9.89 kPa, respectively. In the 0-20cm soil layer, shear strength of soil with two herbal plants were significantly greater than that of the bare land. (4) Soil shear strength had extremely significantly positive correlations with biomass density, root surface area density, and fork density (P < 0.01), and it also had a significantly positive correlation with root length density (P < 0.05). The most important factor of Pennisetum sp. to characterize soil shear strength was biomass density, while that of Paspalum wettsteinii was root length density. It is concluded that, root systems of both herbal plants could improve soil shear strength in the collapsing gully alluvial fan, and Pennisetum sp. was more effective than Paspalum wettsteinii.