Debris flow is a kind of typical water ruins to highways, and it is a dynamic load for highway water ruin. The authors, following the moving law of debris flow and characteristics of highway ruin, put forward reasonable section shape of high - speed drainage trough. Then, according to the energy conservation principle and horizontal throwing theory, the authors established a calculation formula for the throwing distance. The results of the calculation formula given in this paper offer a solution for load calculation and control effect evaluation problems and provide theoretical foundation i to structure design and disease control of debris flow along the highways.