Abstract: Abstract: Xining Basin which is located at the conjunction of the Loess Plateau and the Qinghai-Tibet Plateau is selected as the testing area, and the artificially simulated rainfall test and the washout test are carried out on the slopes with the monocultures of Caragana korshinskii Kom., Zygophyllum xanthoxylon (Bunge) Maxim., Elymus nutans Griseb, and Agropyron trachycaulum (Linn.) Gaertn, separately, and with the mixed planting of C. korshinskii and A. trachycaulum, as well as C. korshinskii and E. nutans. And there are denuded slopes there. Based on the artificially simulated rainfall test, the direct shear test is carried out systematically to analyze the influence of the rainfall on the shear strength and the corresponding shear strength variation regularity of 3 different soil layers. The result shows that before the artificially simulated rainfall test, the cohesion force of the soil in layer a (0-20 cm) mixing planted with C. korshinskii and A. trachycaulum, as well as C. korshinskii and E. nutans is 19.5 and 18.9 kPa respectively, which is larger than that for the soil layer a on the slope planted with the single plant under the same condition. The cohesion force for the soil in layer b (20-40 cm) on the slope planted with vegetation is larger than that of the denuded slope. And the difference of the cohesion force for the soil layer c (40-60 cm) between the planting slope and the denuded slope is not significant. After the artificially simulated rainfall test, the composite cohesion force of soil layer a on the mixed planting slopes is greater than that of the bare slopes. The composite cohesion force amplitude of the soil layer a on the 2 kinds of mixed planting slopes is greater than that of bare slopes, with an increase of 30.65%-56.45%. The increased cohesion force amplitude of soil layer a with the monocultures is 4.17%-38.89%, and that of layer b on the slopes with the mixed planting is 29.17%-34.72%. The cohesion force increased amplitude of soil layer c is significantly less than that of layer a and layer b, which means that the herb roots play a role in soil reinforcement in the shallow layer and the shrub roots play a role of anchorage in the relatively deeper soil layer. The research has theoretical and practical significance in the further study on the mechanism of shear strength improved by plants under the condition of natural rainfall in loess area, as well as in preventing geological disasters such as water and soil loss on slope and shallow landslide.