The systematic failure probability of flood－control system is difficult to evaluate because there is much difference among the risks of components of the system. In order to solve this problem, the concept of systematic failure probability of flood－control system is discussed and two failure models, namely, the step*.by*.step failure model and its modification model, are advanced to evaluate the probabilities of some structures failing at the same time and that the flood loss being greater than a certain amount respectively. Then according to the structural kinds and their protecting areas, the flood*.control system is decomposed to many flood－control structures (such as dams, dikes, and so on),and the failure probability of each is evaluated by applying the system reliability theory. The main failure mode is recognized by using efficient algorithm, and the abovementioned two systematic failure probabilities, which are useful for the risk management and decision making in flood protection, are evaluated.