Abstract: Many researches on the mechanism of earth-rock dams due to overtopping failure and the process of break development have been carried out, but few involves the mechanism of clay core dams. It is necessary to present a numerical model to describe the gradual failure process of the clay core dam so as to evaluate the consequence due to dam break. Based on the characteristics of the clay core dam, the field survey data and model test results, the overtopping failure mechanism and the process of break development of the clay core dam are analyzed, and a numerical model is proposed. The intermittent break expansion phenomenon caused by sliding or overturning of clay core during the scouring process of downstream shell break-induced flows is considered. Finally, the validity of the proposed model is verified by the observation data of a clay core dam with sandy gravel.