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In the mountainous area, the alluvial fan was a good place for human activity, but they are exposed to the potential debris flow, especially in southern of Gansu Province, China. Therefore, the Zuizi catchment and Beiyu River Basin were selected research object. Firstly, the rainfall of different return periods were calculated based on the analysis of local meteorological data. Then, the parameters of the numerical model were obtained by the field sampling and laboratory testing, and the velocity, depth, impact force and intensity area could be simulated by FLO-2D model. In addition, the hazard could be obtained by adopting the hazard matrix. Meanwhile, the visual interpretation method was applied to identify the expose assets of the study area, and the interpretation results were check by using actual investigation, hence, the vulnerability could be calculated. Finally, Hazard and vulnerability were combined to obtain the risk of debris flow disasters. The results show that in the catchment, the maximum velocity of debris flow, the maximum depth of debris flow, the maximum impact force of debris flow and high-intensity area were 6.1 m/s, 7.1m, 14.6×104kN, 0.19km2 under 100-year return period, respectively. The degree of hazard, vulnerability and risk zone were mainly distributed between medium class and high class, and they were accounted for 58.85% and 61.89%, 86.12%, severally. These results were match well with the history disaster events, and this illustrated the process of risk assessment was scientific and reliable. Therefore, the risk of Beiyu River Basin could be obtained based on the process of risk assessment in small catchment. The results indicated that the distribution of hazard, vulnerability and risk class was like to the small catchment, and the proportion of every results were 76.28%, 58.47%, 74.85%, respectively. This study provides local governments with a scientific and evidence base for economic development planning, environmental management, and disaster prevention and reduction. [ABSTRACT FROM AUTHOR]