Abstract: Aiming at the problem of insufficient targeting and timeliness of early warning of flash flood disasters in villages and towns, based on the characteristics of mountain basins in Zhejiang Province, a set of early warning rules system of flash flood affecting villages and towns based on rainstorm threshold was proposed. The system determines different warning periods based on the catchment area of townships, and evaluates the flood control capacity of townships by assessing the disaster water level and corresponding flood frequency of representative sections of townships; The assumption that rainstorm and flood have the same frequency is used to determine the red early warning threshold, and the yellow early warning threshold is determined by comprehensively considering the convergence characteristics and transfer time. Taking Yuhu Town in Wencheng County, Zhejiang Province as an example, this paper expounds the theoretical application hypothesis in typical small watersheds. The research results can provide technical reference for refined flash flood early warning in similar mountainous towns