Abstract: On July 22, 2023, Fuyang Distric of Hangzhou city of Zhejiang Province, was hit by a short-duration heavy rainfall, which induced a flash flood and caused casualties and property losses in the Shanxi River Watershed.To comprehensively understand the flash flood process and reveal the mechanism of flash flood in the affected area, the flash flood characteristics were analyzed from rainfall characteristics, flash flood process and human activities based on the field investigation data.The results show that short-term heavy rainfall is the main cause of the flash flood, with the maximum 1 h rainfall of 100.5 mm and the maximum 3 h rainfall of 150.0 mm at the site, and an over 100-year return period.The hydrological process simulation and flood level analysis of typical cross sections show that the peak flow rate of the basin outlet cross-section reaches 185.2 m³/s, and the flooding process is characterized by a short duration, steep rise and fall, and the water level is rapidly increased to form floodplain floods.The maximum steep rise rate exceeds 1 m/h, greatly limiting the time for residents along the river to avoid danger and transfer.Human activities such as land reclamation have seriously encroached on the flood discharge area of river.The land of residents increased by 160.3%from 2000 to 2022.The water blocking effect of buildings along (across)rivers further hindered flood discharge, exacerbating the scale of mountain flood disasters.