[Objective] The development characteristics of preferential flow for a vetiver slope and the relationship between vetiver roots and preferential flow were analyzed in order to provide a theoretical basis for further study on the influence of plant roots on slope soil stability under varying rainfall conditions.[Methods] Preferential flow on a vetiver slope was studied by using a dye tracer test with simulated rainfall amounts of 30 mm and 50 mm, and plant spacings of 50 cm and 10 cm.[Results] The hydraulic response of the slope soil was greatly increased when vetiver plant spacing was reduced from 50 cm to 10 cm. Under the action of 50 mm of rainfall, the dyeing area ratio and preferential flow ratio for the 10 cm plant spacing were 1.55 and 1.76 times of those for the 50 cm plant spacing, respectively. There was no obvious change in the number of dyeing paths within the topsoil depth range of 0-10 cm. In the topsoil depth range of 10-35 cm, the number of dyeing paths with the 10 cm plant spacing was more than that of the vetiver slope soil with the 50 cm plant spacing. When rainfall was increased from 30 mm to 50 mm, the water flow migrated more quickly into the sloped soil through the dominant channel. The dyeing area ratio (36.11%) and preferential flow ratio (67.25%) with 30 mm rainfall was less than the dyeing area ratio (49.68%) and preferential flow ratio (79.96%) with 50 mm rainfall. The number of dyed paths increased with increasing rainfall.[Conclusion] The root characteristics of vetiver were closely related to preferential flow. When selecting plant slope protection, reasonable plant spacing should be selected to maximize the mechanical and hydrological effects of root slope protection.