By considering the integrity of microcosmic and comprehensive geomorphological features, this article intends to develop a quantitative method for regional risk assessment of debris flow by analyzing the in-depth relations among hazard-forming environments, disaster factors and elements at risk. Taking the debris flows in the Longxi River Basin in China as a case study, an approach based on the hydrological response unit (HRU) is proposed by establishing both a model and system for regional risk assessment to analyze the hazard of debris flows and the exposure and vulnerability of elements at risk. The HRU-based approach consists of 11 disaster factors: topographic slope; relative elevation difference; lithology; channel gradient; fault; disturbed area; rainfall; and the type, location, fragility and economic value of the elements at risk. Specifically, a hazard integrated model is built by considering the hazard factors and their corresponding weights to evaluate the hazard level of debris flow. Through determining the type, quantity, distribution, economic value and fragility of hazard-affected objects in a region, methods are established to determine the exposure and vulnerability of different elements at risk. Validated by using a field study of actual debris flows, a debris-flow risk map is obtained, and the assessment results are in accordance with the actual disaster situation. The analyses show that the distributions of zones with high risk and above (i.e., where hazard densities and economic activities are considerably high) are closely correlated with the topographic and socioeconomic characteristics of the debris flow catchment. These findings suggest that the assessment results provide scientific support for planning measures to prevent or reduce debris flow hazards; thus, the presented method may serve as a pertinent guidance for regional risk assessment of debris flows in both the Longxi River Basin and beyond. (C) 2019 Elsevier B.V. All rights reserved.