Abstract: To face the challenges of flash-flood simulation in ungauged small hilly watersheds,the meter-resolution MIKE FLOOD coupled model was utilized for the Baiyunchuan watershed.This model integrated multi-source topographic data and simulated the full-process of an extreme flash-flood (EFF) triggered by an unprecedented extreme rainstorm.The simulated full-process included catchment runoff generation,channel routing,flash-flood propagation,and disaster inundation.Then,the various disaster mechanisms for each village were illustrated.The results indicate the EFF’s inundation areas expand significantly,inundation depth exceeds twice and maximum peak discharge reaches three to four times that of normal events.The EFF’s flow velocities in the main channel generally exceed 6m/s,and the subsequent overbank flow velocities across the inundation area remain between 2m/s and 4m/s.The study reveals potential inundation risks in traditionally designated “non-hazardous zones” under extreme rainstorm scenarios.The research provides technical support for regional EEF defense.