Abstract: To improve slope stability, the influence of root content in soil on the shear strength of soil is researched. Indigofera amblyantha and purple soil are chosen as research objects. The shear strength of Indigofera amblyantha root-soil composite are researched in the presence of different root contents (0.0 g, 0.1 g, 0.2 g/60 cm³, 0.3 g, 0.4 g, 0.5 g, 0.6 g, 0.7 g, and 0.8 g per 60 cm³). Results show that Indigofera amblyantha root-soil composite obeys the Mohr-Coulomb strength failure criterion. The shear strength of samples with roots is larger than those without roots, which suggests that roots increase the shear strength of root-soil composite. Under the same normal stress, the shear strength increases firstly and then decreases with the increase of root content. There is an optimum value of root content between 0.3 g - 0.5 g per 60 cm³ which provides the maximum shear strength of Indigofera amblyantha root-soil composite. This research offers reference for the ecological protection of slope and purple soil stability in Three Gorges reservoir area.

Key words: ecological protection engineering for slope, root-soil composite, Indigofera amblyantha, purple soil