Abstract: In recent years, the risk of extreme super rainstorm in megacities coupled with big flood in rivers has been increasing, which caused heavy casualties and property losses. It is necessary to carry out joint prevention and control measurements to cope with river flood and urban waterlogging disaster chains. At present, it is an urgent need to integrate multidisciplinary theories and methods of meteorological science, hydrological science, information technology, disaster prevention and mitigation technology, to solve the key scientific and technological issues. The key scientific issue is the combination mechanism of river-urban flood and the risk transmission rule of disaster chain. Besides, there are four key technological issues : ① To integrate the river and urban meteorological-hydrological observation facilities into a collaborative one to monitor and forecast the flood risk. ② Intelligent early warning and directional message broadcasting technology for river flood and urban waterlogging disaster chain. ③ Coupled simulation of river-urban flood and demonstration of joint prevention and control scenarios. ④ Intelligent decision-making technology for river-urban joint flood control and emergency plan preparation. Based on the collaborative observation of meteorology and hydrology, social and economic multi-source information convergence analysis, and disaster deduction model, an emergency command and decision support system could be built for joint prevention and control of river-urban flood. The system highlights real-time, dynamic assessment, precision and intelligence to support the whole-process river-urban flood regulation. It could provide technical support for flood forecasting, early warning, scenario-demonstration and emergency plan preparation in megacities.