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In view of the spatial scale of threat and potential risks caused by debris flow under four extreme rainfall conditions, risk assessments of debris flow hazard in Maiduo Gully, Chaya County, Tibet were carried out, using a single building as an evaluation unit. Assessments include the risk division, vulnerability analysis and risk assessment of debris flow. This paper describes a whole workflow of a refined risk assessment. Firstly, based on a three dimensional geological model acquired from UAV, the FLO⁃2D software was used to simulate the process of debris flow outbreaking in Maiduo Gully under four extreme rainfall conditions(10%,5%,2%,1%),and the risk zones were divided. Then, analyzed the vulnerability of buildings and population in the affected area according to on site data. After that, took a single building as the evaluation unit, risks and vulnerability were normalized, and to eventually get the risk zones of building and population in study area, overlay analysis was applied on ArcGIS Desktop. These evaluation results are able to provide scientific reference to the works of local geological disasters and risk control, and the evaluation workflow can be used as a reference for future refined quantitative risk assessments of debris flow hazard as well. [ABSTRACT FROM AUTHOR]