Abstract: To clarify the morphological characteristics of soil preferential flow and the effect of plant roots on its formation, plants from the typical vegetation types of an artificial woodland (Leucaena acacia) and a dry watershed grassland (Heteropogon contortus) of Yuanmou County, Jinsha River were selected as the experimental objects. Based on the staining and tracing method combined with Photoshop CS5 and the Image-Pro Plus 6.0 image processing technology, we analyzed the morphological and distribution characteristics of soil preferential flow under the two planting types and examined the effects of plant roots. We found significant difference in soil preferential flow dyeing area between the woodland and grassland species, and the overall variation trend of the forestland dyeing area ratio decreased with increasing soil depth. The dyeing area of the grassland decreased monotonously with the increases of soil depth. The occurrence degree of soil preferential flow in forest was higher than that of grassland. Root systemaffected the formation of soil preferential flow. At the root diameter ranges of 0≤d≤5 mm and d>10 mm, root length density of the woodland showed a monotonous decreasing trend with increasing soil depth, while in the root diameter range of 5 mm<d≤10 mm, it fluctuated in the 30-40 cm soil layer. Root length densities at all the root diameter levels of desert grassland were negatively correlated with soil depth. The dyeing area ratio of the woodland was significantly correlated to root length density in the 3 mm<d≤5 mm root diameter range, while that of the grassland was significantly correlated to root length density in the d≤3 mm range. The dyeing area ratio of both land types showed significant correlation with root weight density in the range of 1 mm<d≤3 mm and with root surface area of d≤1 mm, but did not correlated with root length density, root weight density, and root surface area in the root diameter range of d>5 mm. The overall change trend of soil preferential flow dyeing area of two vegetation types in the study area decreased with increasing soil depth. Plant root system was closely related to the formation of soil preferential flow. Fine roots could promote while coarse roots may retard the formation of preferential flows.