Abstract: The flood response of a watershed is a systematic product of the combined effects of climatic conditions and underlying surface characteristics. The Qingshuihe River in Mentougou District(west) and the Qingshuihe River in Miyun District(north) in Beijing showed significantly different flood responses in the "23.7" and "25.7" extreme rainstorm respectively at similar watershed scales. Construct a vertical mixing and geomorphic unit line(GIUH) coupled runoff generation and convergence model, combined with digital elevation models, soil hydrological characteristics, topography, lithology, and measured rainfall runoff data, to study the runoff response laws and differences between the two watersheds from the perspectives of underlying surface conditions and runoff generation and convergence mechanisms. The results show that the terrain features of the two watersheds are similar, but the density of the Qingshui River network in Miyun District is 4.4 times that of the Qingshui River in Mentougou District, and the convergence path is concentrated; The runoff parameters show that Qingshui River in Miyun District is more prone to over seepage and runoff, while Mentougou District has stronger infiltration capacity; The GIUH unit line indicates that the peak of the Qingshui River in Miyun District is sharp and thin, and the peak appears early. The study reveals that river network structure, soil lithology, water storage and infiltration capacity are the main control factors of rainstorm flood response in the basin, and the results can provide a reference for mountain flood forecasting.