Management on the jointly discharging of overtopped cofferdam is a typical semi-structured problem. So a decision support system is designed to address the management on the jointly discharging of overtopped cofferdam. This paper introduces systematically the structural framework, design principle, decisionmaking mode, and fundamental function of CDDSS. The system consists of four components： database subsystem, model base subsystem, graphics base subsystem, method base subsystem, and user-interface. A CDDSS is a man-machine interactive system,which integrates computer science and technology with the theory and method of systems science, management science and the theory and method of hydraulic and electric engineering. The case study illustrates that the friendly window-based user interface of the system enables the decision maker/user to take full advantage of the capabilities of the system in order to make effective real-time decisions.