Plant slope protection measures play an important role in the improvement of loess slope stability and the restoration of ecological environment. Here, the shear strength of undisturbed Robinia pseudoacacia root composite soil was measured by a large direct shear device, and the reinforcement effects of Robinia pseudoacacia root system with different growth years on loess slopes were analyzed by the geotechnical engineering numerical calculation software FLAC3D. The results show that the plant roots can improve the shear strength of root composite soil significantly because the root system plays the role of reinforcement anchoring in the soil, and the shear strength of root composite soil conforms to Mohr-Coulomb strength criterion. The shear strength of root composite soil increased with the increase of root growth years, the cohesion (c) and internal friction angle (φ) of 3-7 a Robinia pseudoacacia undisturbed root composite soil were 44.46- 88.91 kPa and 23.89°-28.85°respectively, and 7 a Robinia pseudoacacia root composite soil had the highest shear strength, its c and φ were 211% and 31% higher than rootless soil respectively. Plant roots can strengthen the loess slope and improve its stability, make it less prone to displacement; however, due to the shallow depth of the soil reinforced by the Robinia pseudoacacia root system, the reinforcement effect on the slope is limited. Compared with the loess slope without the planting of Robinia pseudoacacia, the improvement of the safety factor of the 7 a Robinia pseudoacacia root system on the slope stability was only accounted for 9%. The research results can provide a technical support for the design and construction of control measures of plant slope protection such as Robinia pseudoacacia.