Abstract: The flood safety of dams is subjected to various stochastic factors.With the elapse of time,those factors having negative impact on the flood control safety are of obvious time-varying characteristics.The time-varying characteristics of flood control of dams due to this effect,both the impact of mild change effect and steep change effect,are demonstrated,then the time-varying characteristics of various stochastic factors during the dam ageing as well as their effect on flood safety are analyzed.With regard to the following 3 kinds of stochastic factors during the long-term operation of dams:settlement of dam top,deposition of sediment and failure of flood releasing gates,the quantitative evaluation methods,that is,attenuation function approach,comprehensive analysis approach and Bayesian approach,are put forward.