The observed data shows that the resistance of debris flow is higherthan that of water under the same hydraulic condition. According to theanalysis on different effects of coarse grains and fine grains for resistan-ce. This paper thinks that the reason causing increase of resistance ofdebris flow is the bed load in debris flow.In order to prove this idea, wehave analysed the relation between extra resistance causing bed load andincrement of resistance of debris flow. The result shows that the incre-ment of resistance increace with the extra resistance increasing.