In order to explore the existence of preferential soil flow in the Loess Plateau in western Shanxi Province and the effects of different infiltration water conditions on the degree of preferential flow development, Robinia pseudoacacia plantation in Caijiachuan small-watershed were taken as the study object, and a combination of field dyeing experiment and indoor image processing technology was used to obtain the morphological images of soil vertical profile dyeing and to extract the characteristic indicators. Six characteristic indicators, including the matrix flow depth, the dyeing area ratio, the preferential flow ratio, the length index, the average maximum dyeing depth and the coefficient of variation of staining morphology, were chosen to measure the development of preferential flow across various infiltration water scenarios based on the root mean square difference decision method. The results showed that: 1) The phenomenon of preferential flow existed in the process of soil water movement of Robinia pseudoacacia forests, with the increase of infiltration water, the preferential path began to develop laterally, and the phenomenon of preferential flow side infiltration was obvious. 2) The dyeing area ratio showed a non-linear decreasing trend with the increase of soil depth, and there was non-uniformity in the process of water infiltration. At an infiltration rate of 75 mm, the preferential flow ratio reached 47.11% with the highest level of preferential flow development. 3) The average maximum dyeing depths under the three infiltration water conditions were 25.33, 32.20 and 38.87 cm, respectively, which were positively correlated with the magnitude of infiltration water, indicating that an increase of infiltration water can directly promote the movement of soil water to deeper layers and facilitate the opening of preferential path. 4) There was a positive correlation between infiltration water and the preferential flow evaluation index, with the preferential flow evaluation index of 75 mm (0.81)> 50 mm (0.39) > 25 mm (0.36),, and the increase in infiltration water led to a higher degree of preferential flow development.