Based on the hydrodynamic model and established a mathematical model of debris flow drove by rainfall and river runoff, and took topography, geological formation, plant mulching and demographic distribution as the impact factors. Five cities of Guangxi have been selected as the research area, the rivers information has been generalized by the river net codification. Based on the rough set theory, a synthesized weight calculation method of the impact factors has been presented, the weight of these factors could be determined objectively, thus a risk analysis model of debris flow has been established. Indeed, the numerical simulation results have been validated in statistical results of geological hazard distribution and the debris flow event of Guangxi, it shows that the mathematical model of debris flow based on hydrodynamic theory is reasonable and feasible.