Abstract: Considering the potential failure modes in levee engineering,the mathematical models of hydrological failure risk and slope slide instability risk for levees are improved.Computing methods of conditional probability functions for overflowing failure risk,seepage failure risk and slope slide instability risk are described.A mathematical model of integrated failure risk for levees is set up.Meanwhile the calculation method of failure risk under over-level flood conditions is presented.According to these proposed models and methods,the risk analysis of External Qinhuai River levees of Nanjing under over-level flood conditions is carried out,and the failure risk factor of the levee under over-level flood conditions is found to be 0.57%.