The effective implementation of the "grain-for-green" project has rapidly restored vegetation on the Loess Plateau, which will inevitably trigger the changes in plant roots and soil physical and chemical properties, and further affect the shear strength of root-soil system. To investigate the potential impacts of vegetation restoration age on shear strength of root-soil system, six abandoned farmlands with different restoration age (5, 12, 20, 27, 37 and 46 years) and a slope farmland were selected as testing sites in Zhifanggou small watershed in Ansai, Shaanxi province. The in-situ shear tests were conducted in five different soil depths (0—10, 10—20, 20—30, 30—40, 40—50 cm), and root characteristics and soil physical and chemical properties were measured simultaneously. The results showed that the average shear strength of 0—50 cm soil layers increased as a sigmoid curve from 51.80 J/m² to 124.01 J/m² with the increasing of vegetation restoration age. Compared with slope farmland, the shear strength of root-soil system increased by 2.5%, 54.6%, 48.7%, 86.5%, 139.4% and 129.3% for abandoned farmlands with different restoration ages, respectively. The root density decreased significantly with the increasing of soil depth in the abandoned farmlands, which induced decrease in the shear strength of root-soil system. The shear strength of root-soil system was logarithmically positively correlated with organic matter content, root length density and root mass density, and linearly positively correlated with aggregate stability and root density. Path analysis showed that aggregate stability, root mass density and organic matter content were the key factors affecting the shear strength of root-soil system. The results could provide a theoretical basis for evaluating water and soil conservation benefit and ecological service functions of vegetation restoration, and revealing the dynamic mechanism of changes in shear strength of root-soil system with the vegetation restoration age.