Abstract: Many studies can be found regarding river flood and urban flood induced by heavy rainfall. However, o few are focused on the compound flood caused by them. The mechanism of the compound flood process and the measures that could be taken to mitigate its impact need a systematic research. By taking the river flood happening in August 2020 in downtown city of Chongqing, the flood characteristics including the flood area and depth have been simulated by using the proposed modeling system CUFM（Compound Urban Flood Model）. By comparing the results by using MIKE 21, the flood extent of the latter is more serious and it cannot reproduce the overflow flood through the pipeline.Taking the Dashaxi Region in Nan’an District of the downtown city as the study area, the compound flood process and its rationale have been analyzed numerically. With the increase of the rainfall return period, the flood area also expands gradually. The reason can be partially attributed to the jacking at the outfall and pipe overflow. Besides, the topography decreases from the southeast to the northwest which forces the mountainous runoff flowing to the low-lying area, but due to the river flood, the runoff cannot go into the river or be collected by the surcharged pipe system. Finally, the response relationship between the variation of flood volume and the rainfall pattern under the impact of the river flood has been simulated and discussed. It shows that under the circumstance of small rainfall return periods, the rain peak occurrence does not affect the flood volume but the rain with early peak occurrence would result in a rapid flood increase. With the increase in rainfall return period, the rain peak has a great impact on the flood volume, the rain with a later peak occurrence would produce higher flood peak and the flood peak comes later. The conclusion of the research indicates that in case of small rainfall return period, precautions should be taken against the rain with early peak occurrence which may speed up the increase in flood, and for large rainfall return period, preventive actions need to be taken to mitigate the impact brought by the rain with later peak occurrence.