In recent years, frequent incidents of urban water logging have highlighted the problems of urban flood control. Based on the brittleness theory of completed system, the paper used brittleness structure model,and combined fault tree analysis(FTA) methed to extract brittleness factors, and constructed the model of urban flood control system.Therefore, the nature of failure of flood control system function is system crash.Then,by aid of theory of risk entropy, the paper proposed risk calculation method of urban flood control system and introduced system safety coefficient to determine the trend of system crash.Finally,it used MIKE11 hydrodynamic simulation software to carry out hydrodynamic simulation.Taking Suzhou area for example,it validated the effectiveness of theoretical method.The result can provide the reliable data for city flood warning of Suzhou area.