To investigate the stability mechanism of vegetations on sandy slope under rainfall, a comparative experiment between “exposed slope” and “vegetated slope” was conducted by indoor rainfall model tests. Then, three kinds of selected fast-growing vegetations (Chinese holly, Bermuda-grass, and festuca arundinacea) were planted on sandy slope to further analyze vegetation slope protection. It was found that the growth curves of the vegetations were nearly S-type during plant growth, and the root depths were about 1.7-2.1 times of the vegetation heights. Under the same rainfall condition, the steady state time of vegetated slope was two times more than that of exposed slope. The global failure of exposed sandy slope had the characteristics of suddenness and linearity. Root architectures of vegetations played an important role in slope stability. The rainfall model test results show that the overall stability of the sandy slope with Bermuda-grass was the best, followed by the Chinese holly, and then the festuca arundinacea, whose root architectures were inverted triangle, uniform, and cone, respectively. Meanwhile, the growing root could bind the surrounding fine sand particles by releasing viscose materials. Consequently, the roots of vegetations were intertwined together to form a root net, and a compacted root-soil composite protective layer structure of 22 cm in thickness was formed on the slope surface. Additionally, Chinese holly with uniform root architecture could effectively prevent the occurrence of the local flow-type failure near the slope toe in the case of ponding water at the slope base, which can help to explore new bionic support technologies.