Abstract: The key mechanism of debris flow initiation in a channel is the process of shear force over-coming bed friction. In this paper, the critical equilibrium condition of particle deposit is established by limit equilibrium analysis, which shows the stability of deposit depends on slope angle, internal friction, deposit height and water discharge. Then through series analysis of experimental data in literature, a critical velocity criterion for initiation of gully debris flow is obtained. The criterion is in good agreement with two typical series of experimental results of debris flow, which might provide a useful tool for gully debris flow initiation.