In order to study the influence of vegetation roots on the permeability of weak expansive soil, through variable head permeability tests of weak expansive soil samples with different root content densities, the law of permeability of weak expansive soil samples changing with root content is analyzed, and use SEEP/W software to analyze the volumetric water content at different positions of the slope caused by rainfall. The result shows: the permeability coefficient of the sample at standard temperature will also increase When the root density of the vetiver root system increases. The greater the permeability coefficient of the sample at standard temperature. During the rain period, the increase in volumetric water content is proportional to the root content and elevation. After the rain stopped, after the rainfall ceases, the rate of decrease of the volumetric water content is inversely proportional to the root content, and the rate of decrease is proportional to the elevation.