Abstract: There are many reasons for the burst of rockfill dam,such as: di ff erent areas,different geology,different drainage areas,different dam types and so on.Based on the accident tree principium,we analyze the relations of dif ferent events and burst of rockfill dam,the relations of forming the model eve nt tree structure,and establish a mathamatical model of burst probability of roc kfill dam.To supply a theory platform for developing a software of rockfill dam safty estimation.