Abstract: After the Wenchuan earthquake, many loose solid sources accumulated in the channel, which increased the probability of debris flow . It was difficult to calculate the dynamic reserves of debris flow sediment source accurately. Based on field investigation, data collection and laboratory model test, this paper introduced the fractal theory to quantitatively describe the complex soil particle size composition with fractal dimension, and studied the erosion regular of different deposits in channels under different rainfall effects. A dynamic reserve evaluation model with rainfall intensity and fractal dimension as double influencing factors was established. The results show: Coarse-grained soil is not easy to start, but under sufficient hydrodynamic conditions, erosion will be multiplied; When debris flow occurs, the erosion change and total erosion scale of "fine-grained on coarse-grained soil" are small, and which is beneficial to the stability of channel; The erosion phenomenon of "coarse-grained on fine-grained soil" is similar to that of coarse-grained soil, but the rainfall threshold of large-scale debris flow is lower than coarse-grained soil; For materials source in the channel, the order of erosion effect is headward erosion > shear erosion > lateral erosion > subsurface erosion; The formula fitted in this paper is suitable for the wide and slow channel of debris flow, but it has some limitations for the narrow and steep channel.