Abstract: Due to the urban rain and heat island effect, the extreme rainfall in Beijing urban area continues to increase,and the urban watershed suffers from the double impacts of river flooding and regional flooding, which is extremely difficult to manage. This paper takes the North Canal basin above Yangwa Gate as the research area,based on meteorological and hydrological medium and long-term rainfall prediction results, combined with historical typical storm flooding change patterns and possibilities, and carried out flood scenario construction and simulation based on bottom-line thinking to quantify the risk of storm flooding. The results show that the rainfall in 2021 flood season is slightly more than the same period of the meteorological norm, which is a normal partial drought year. The large proportion of impervious surface under the urban area of Beijing leads to highly correlated rainfall floods, so that water accumulation in the urban area is frequent when extreme rainstorms occur. Considering the most unfavorable scenario, it is expected that the flood peak on the Beiguan Gate will exceed 20 year frequency, and 149 waterlogging locations may appear within the sixth-loop of the city. Pre-discharge, controlled-discharge and staggered-peak dispatching measures of the North Canal should be strengthened, and emergency units should be deployed in advance at severe waterlogging points in the eastern and southeastern parts of the urban area of Beijing and at contiguous waterlogging points in the north, and the results of the study can provide a reference for urban flooding defense in 2021.