Vegetation can effectively improve the stability and erosion resistance of shallow slope. In order to make a preliminary quantitative assessment of dynamic changes of vegetation slope consolidation under natural conditions in the loess hilly and gully region where the shallow landslide prone to occur, taking Robinia pseudoacacia, the main afforestation tree species in the loess hilly and gully region as an example, the dynamic changes of roots mechanical and hydrological effects of vegetation slope consolidation under natural conditions in 2017 were studied. The roots cross-sectional area distribution was investigated by digging the root section, the roots tensile strength was tested by single root tensile test, and the roots additional cohesion of R. pseudoacacia was calculated by using the Wu model. Field surveys were conducted to collect the parameters related to the hydrological effects of vegetation and the research plot. Laboratory infiltration experiment and soil centrifugation experiments were conducted to obtain the soil hydraulic parameters. Then the water transport software Hydros-1D was applied to do the transient analysis of the water content of the R. pseudoacacia slope. Finally, the stability of the slope was analyzed based on the infinite slope model under variable saturation conditions, and the dynamic contribution rate of roots mechanical and hydrological effects of R. pseudoacacia to the slope stability were calculated. The results showed that: (1) The mechanical effect of R. pseudoacacia roots on the stability of shallow slope at the depths of 20, 40 and 60 cm increased by 196.56%±28.08%, 106.74%±13.19% and 20.61%±2.43%, respectively. At the same time, the contribution rate of root mechanical function also had some fluctuation. (2) During the growth period of R. pseudoacacia, R. pseudoacacia canopy intercepted 70.2 mm rainfall, transpiration changed the soil water content and suction stress of the slope, so the stability of shallow slope was effectively improved. (3) The contribution of the roots mechanical action existed throughout the year, and the volatility was less than that of hydrological contribution rate. However, the contribution of hydrological action mainly occurred during the growth period of R. pseudoacacia, and most parts of the peak values of hydrological contribution rate were larger than those of mechanical action. The results further revealed the dynamic changes of R. pseudoacacia slope stability in the hilly and gully region of the Loess Plateau under the natural conditions.