Abstract: The stability of mountain stream bed is directly related to the effects of artificial step-pool system on mitigation of riverbed incision and control of debris flow. In this paper four cases of artificial step-pool system constructed in 2006—2010 are analyzed to figure out the control effect and the causes of destruction of the system. The step-forming boulder is analyzed as an individual object and forces on the boulder is derived. A new model of step-pool stability is obtained, which can be used to analyze the impact of the incoming discharge and scour angle. The step stability primarily relies on the size of step-forming boulders, bed slope, flow discharge, and scour angle. The peak discharge in flood season and scour of downstream step are the fundamental causes of step failures. The stability of artificial step-pool system during flood peak is the key for long term use of the system.