The effect of plant slope protection is strongly associated with the root architecture in soil. In this paper, the water?fertilizer combination method was used to regulate the plant root configuration, and the influence of the regulated plant root architecture on the slope soil strength was studied. In terms of the practice of highway slope engineering, a field fill slope for test was made. Vetiver was selected as slope protection species, and 9 groups of water?fertilizer combinations were designed to regulate the root architecture of plant in slope soil. After 10-months growth, the roots of plants on the slope were counted by cross layered excavation. It was found that the root content increased first and then decreased with the depth of soil layer, which accorded with Gauss curve. In control area with water?fertilizer combination, vetiver proliferated a large number of secondary and tertiary roots, which increased root content and root surface area density. The proportion of roots in the upslope direction was higher than that in the downslope direction, reaching 60%?66%. The shear test of undisturbed soil in different regions shows that the root content and root surface area density are important factors affecting the shear strength of soil. Regression analysis shows that there is a linear relationship between soil cohesion and root surface area density. The calculation based on Wu’s model shows that under natural growth conditions, the increment of shear strength of vetiver to slope soil is 5.28 kPa to 8.62 kPa, while under conditions of water?fertilizer combination, the angles between most of secondary and tertiary vetiver roots and the vertical are greater than the slope angle, which makes the shear strength of vetiver on slope soil increase by 17.59 kPa to 33.97 kPa. This research shows that water?fertilizer combination method can be used to regulate the root architecture of plants to strengthen slope soil, which provides theoretical and practical basis for improving slope soil strength and preventing slope soil erosion.