Abstract:Cities are highly concentrated areas of socio-economic and human activities and are extremely vulnerable to flood disaster losses. Urban flood disaster assessment and law analysis are the basis of flood management and disaster reduction. At present, there are corresponding threshold standards for the classification of rainstorm levels. However, due to the comprehensive influence of urban underlying surface conditions and drainage systems, there is certain regional differences between rainstorm levels and flood level. Taking Shanghai City as the study area, historical rainstorm data of representative stations from 1960 to 2019 were selected to explore the relationship between rainstorm and flood threshold levels in Shanghai City. Cumulative precipitation indicators of 24h, 48h and 72h were selected to calculate the empirical frequency of precipitation in different time scales. Combined with the threshold standard of flood level return period, the quantitative identification of flood based on the precipitation indicators of different time scales in Shanghai was studied. On this basis, the Morlet wavelet was used to analyze the laws of flood and waterlogging in Shanghai to provide quantitative basis for flood assessment and disaster prevention and mitigation in Shanghai. The results of the study show that: ① the average of 24h accumulated precipitation of Shanghai for return period of 50 years is 125.3mm, the average of 24h accumulated precipitation for return period of 20 years is 99.7mm, and the average of 24h accumulated precipitation for return period of 5 years is 59.4mm.②In Shanghai, the extremely major or major floods occur once every two years on average, and moderate or above floods occur 2.17 times a year on average. The occurrence of flood has obvious seasonal patterns, mainly in summer and autumn, accounting for 65% and 28% respectively.③There is no obvious periodic pattern in the frequency of moderate or above flood in Shanghai. This study provides quantitative basis for flood assessment and disaster prevention and mitigation in Shanghai City.