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Preferential flow is the phenomenon of water and solute rapidly transport through preferential flow paths such as macropores, wormholes and root holes. Studying the development degree of preferential flow has a certain reference value for efficient use of agricultural fertilizer and prevention of groundwater pollution. With the purpose of evaluating the development degree of preferential flow under different slope farmlands in Chongqing purple sandstone area, the dye tracer experiments were designed in three sample regions (pumpkin, citrus and maize). The morphological and statistical methods were used to extract parameters from the image of soil profile in three slope farmlands. Multi-index evaluation method was used to determine the preferential flow indicator PFI. Combined with fractal theory in geometry, the development degree of soil preferential flow was quantitatively evaluated. Six characteristic preferential flow indexes (staining area ratio of preferential flow area, uniform flow infiltration depth, preferential flow fraction, length index, peak index and coefficient of variation) revealed that the preferential flow of pumpkin and citrus was higher than that of maize;the order of preferential flow indicator PFI from large to small was pumpkin (0.88), citrus (0.77) and maize (0). Box-counting dimension calculation of wetting front curve can characterize the non-uniformity of soil water infiltration. The larger box-counting dimension was, the more irregular shape of wetting front curve and the higher degree of preferential flow development were. Fractional dimension of wetting peak trace calculated by Fractal Fox 2.0 software basically got the same result, which showed that it was feasible to evaluate the development degree of soil preferential flow quantitatively. Fractional dimension with advantages of simple operation, low cost and high accuracy was suitable for field scale, which can enrich the study method of soil preferential flow.