Abstract: Vegetated slope protection, as an ecological technology for controlling shallow landslides, has attracted increasing attention in recent years. Its primary mechanism relies on the mechanical and hydrological reinforcement effects of plant roots on slope soil, with the frictional characteristics of the root-soil interface being essential for understanding the mechanical reinforcement mechanism. This study focused on completely decomposed granite, a soil type commonly found in South China. Single root pullout tests were conducted using a self-developed device to investigate the influence of stress state and matric suction on the frictional behavior of the root-soil interface. The results indicate that: (1) The shear stress-displacement curves of the root-soil interface exhibit three distinct stages: an increase in static friction shear stress, a rapid decrease in dynamic friction shear stress, and a gradual decline in dynamic friction shear stress.(2) The frictional strength at the root-soil interface shows a significant positive linear relationship with confining pressure. As the confining pressure increased from 0 kPa to 75 kPa, the peak and residual frictional shear stresses increased by 219.3% and 222.1%, respectively.(3) Moisture content and matric suction in unsaturated soil significantly influence the frictional strength of the root-soil interface (P< 0.05). A strong linear relationship was observed between both peak and residual frictional shear stresses and matric suction, expressed by the equations y=0.111x+8.03 and y=0.14x+2.52, respectively. These findings provide a theoretical basis for studying the reinforcement effects of plant roots and supporting ecological prevention strategies for shallow landslides in South China.