This paper focuses on a model that predicts the runout and velocity of the Dongwopu debris flow in Tianjin, China. Based on the law of energy-conservation, the established model primarily considers potential energy and energy loss in debris-flow movement. The present study discusses several stresses that could cause energy loss for debris flows, and the friction energy loss is emphasized. Ultimately, the analytical expressions of runout and velocity are established. The calculation results show that the velocity of the Dongwopu debris flow is 14 m s-1. At this velocity, the runout distance will not cause a threat to residents and engineering facilities in the area.