In this paper,the constitution of valley flood control system,influence factors and their analysis methods are studied in viewpoint of risk analysis.The contents include the estimations of system failure probability and flood loss and the effect of nonstructural measures on decreasing system risk.In connection with the features of flood defense engineering of the middle Yangtze River,the constitution of flood defense system is investigated.The study result shows that through the system risk analysis,the safety,economy and sociality of flood defense engineering may be combined one another as a synthesis for serving the establishment of flood control decision supporting system