Abstract: Under the condition of many years of operation, the existing water conservancy projects are faced with problems such as structure aging and operation condition change, which cannot meet the safe operation under the design conditions, causing the problem of cooperative scheduling between the existing water conservancy projects and the new projects. In order to solve the problem of cooperative flood control and dispatch of new and old water conservancy projects, this paper collects historical flood data, and uses Pearson-Ⅲ distribution and normal distribution to fit the peak flow distribution respectively. Monte Carlo method is used to randomly select the pseudo-random numbers, which are more than the current design standard, and then uses the peak-ratio amplification method to design the 15 d flood process. The characteristic flows of the new and old projects are combined to form different operating conditions, and the risk of collaborative flood control under various working conditions are calculated. This paper takes the pre-post aqueduct（the old aqueduct） in Ganfu Plains of Jiangxi Province and its extension project, inverts siphon（the new aqueduct）, as the research objects. After the characteristics of the projects have been analyzed, the characteristic flows of the projects are presented as the minimum operating flow under the current safety control and the design flow of the old aqueduct, and the design flow and increased flow of the new aqueduct. The results show that the flood risk calculation results based on the P-Ⅲ distribution are about 40% higher than that of the normal distribution, indicating that the flood peak distribution using P-Ⅲ distribution can simulate more adverse flood conditions. When the P-III distribution is used to fit the flood peak discharge, the risk rate of the old aqueduct is 23% when the new aqueduct is operated according to the design standard, while the risk rate of the new aqueduct is 8%when the old aqueduct is operated according to the design standard. It indicates that the risk of old aqueduct is higher when aqueducts work together to control floods.