On the basis of 182 gravelly soil samples taken from debris flow source area

the particle size distribution are obtained through laboratory test

fractal dimensions of samples are calculated by fractal theory. Analysis of calculated results show that the gravelly soil samples are mainly one dimension fractal; and it account for 88.46% of the total samples

the value of one dimension fractal is between 2.250－2.798. Based on the calculated fractal dimension

soil samples are configured; and self-made constant head device with controllable waterhead are used to conduct penetration experiment. The experimental results indicate that the correlation between permeability coefficient and fractal dimension is significant. When dry density equals 1.8 g/cm3

the correlation of permeability coefficient and fractal dimension is best; and the relationship between permeability coefficient and fractal dimension shows obvious power function using multiple regression analysis under different density conditions. Moreover

under same fractal dimension (condition)

the permeability coefficient decreases with the increase of density; the power function relation between fractal dimension and permeability coefficient is obvious when fractal dimension ranges from 2.450 to 2.600. This study results can provide theory basis for future critical rainfall research; meanwhile

these can improve the universality and accuracy of the existing forecasting models.