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 ofCaragana korshinskiiKom.,Zygophyllum xanthoxylon (Bunge) Maxim.,Elymus nutans Griseb, and Agropyron trachycaulum (Linn.) Gaertn, separately, and with the mixed planting ofC. korshinskii andA. trachycaulum, as well asC. korshinskii andE. 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 withC. korshinskii andA. trachycaulum, as well asC. korshinskii and E. nutans is 19.5 and 18.9 kPa respectively, which is larger than that for the soil layera on the slope planted with the single plant under the same condition. The cohesion force for the soil in layerb (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 layerc(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 layera 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 layerc is significantly less than that of layera 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.%该研究以位于青藏高原东北部黄土区的西宁盆地作为试验区，采用柠条锦鸡儿（Caragana korshinskiiKom.）、霸王（Zygophyllum xanthoxylon (Bunge) Maxim.）、垂穗披碱草（Elymus nutansGriseb.）、细茎冰草（Agropyron trachycaulum (Linn.) Gaertn.）的单播种植，以及柠条锦鸡儿分别与细茎冰草、垂穗披碱草混播种植等2种方式，开展了原位模拟降雨试验并采用直接剪切试验，系统分析了模拟降雨试验前后试验区边坡表层以下3个不同位置处复合体抗剪强度大小及其随深度变化规律。结果表明：模拟降雨试验前，由柠条锦鸡儿与细茎冰草、垂穗披碱草混播的边坡a层（0～20 cm）根-土复合体黏聚力分别为19.5、18.9 kPa，均较相同条件下单播边坡a层复合体黏聚力大；混播种植边坡各层根-土复合体黏聚力均较模拟降雨试验后裸坡 a 层土体黏聚力大，其中柠条锦鸡儿与细茎冰草混播边坡、垂穗披碱草混播边坡模拟降雨试验后，a层复合体黏聚力分别较裸坡a层黏聚力的增幅为30.65%～56.45%；柠条锦鸡儿、霸王、垂穗披碱草、细茎冰草单播边坡b层复合体黏聚力增幅为4.17%～38.89%；柠条锦鸡儿分别与细茎冰草、垂穗披碱草混播边坡b层黏聚力增幅为29.17%～34.72%；边坡c层土体黏聚力增幅相对不及a层、b层显著，反映了草本起到浅层加筋作用，灌木则起到深部锚固作用。
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