[Objective] The development characteristics of preferential flow in slope farmland with different soil thicknesses were studied, in order to provide a basis for improving the irrigation efficiency and preventing and controlling the non-point source pollution. [Methods] The granite hilly slope cropland in Mengyin County, Shandong Province, which is located in the mountainous region of Northern China, was taken as an example. The samples with soil depth of 30 cm and 50 cm were selected, and the field staining tracer test method and indoor image processing technology were used to analyze the influence of soil thickness on the development characteristics of preferential flow. [Results] When the soil thickness was 30 cm and 50 cm, the average depth of substrate infiltration was 11.8 cm and 11.9 cm, respectively, and the average preferential flow ratio was 28.2% and 29.5%, respectively, there was no significant difference between the two depths. When the soil thickness was 30 cm, substrate flow was dominant in the soil layer above 9.9 cm, and the development depth of preferential flow ranged from 9.9 to 27.0 cm. When the soil thickness was 50 cm, matrix flow was dominant in the soil layer above 6.1 cm, and the development depth of preferential flow ranged from 6.1 cm to 39.5 cm. Under the condition of 50 cm soil thickness, the average maximum infiltration depth of preferential flow was 34.6 cm, the average length index was 192%, and the average coefficient of variation was 87.7%, which were significantly higher than those under the condition of 30 cm soil thickness. However, there was no significant difference in the non-uniformity coefficient of maximum infiltration depth between the two conditions. [Conclusion] The difference of soil layer thickness did not affect the substrate infiltration, but hindered the development of preferential flow to deep soil layer.