The debris flow impact size is one of the most important parameters for designing debris flow hazards prevention project

but so far

there is a dispute of the calculation of debris flow impact.This paper analyzes the different debris impacts on the debris dam under different densities of debris flow

channel slope and the angle of debris dam to meet the water through the indoor flume experiment testing.This research shows that the main factors influencing the debris flow impact size is debris flow density

channel slope

and debris flow velocity;Other factors are equal.Debris flow impact size decreases with increasing of debris flow density

increases with the increase of channel slope;Debris flow density and the channel slope mainly reflect the influence of the change of debris flow velocity.We deduces a new formula of debris flow impact size.The formula shows that the debris impact size has a linear relation with debris flow velocity

and the linear coefficient varies because of the debris flow density.This paper compares the differences of the new formula with the original formula and illustrates the practical value and provides a technical support to new designing prevention projects of debris flow.