Since the effect of excess pore pressure wasn't considered in pervious formula for the critical slope of the initiation of debris flow. The initiation of debris flow due to excess pore pressure on the channel bed is explored in this paper by employing theoretical analysis. A general formula considering the effect of excess pore pressure is proposed, which agrees well with other formulae while the pore pressure term is dropped. Once the excess pore pressure exists, the deposit materials on the channel bed is more prone to the occurrence of debris flow no matter where the water level is. The formula proposed in this paper agrees well with experimental data.