Abstract: Overtopping failure leads to more than 50% of the broken earth-rock dams in China, so it is necessary to study the mechanism of overtopping failure and the process of break development in order to forecast the discharge hydrograph of break and the consequence due to dam-break. In this paper, based on the field survey data and model test results, the overtopping failure mechanism and process of break development of earth-rock dams are analyzed, and a numerical model which describes the process of break development of earth-rock dams is put forward. The model has the following characteristics:the sediment transport formula of high-velocity flow considering the physical and mechanical properties of the dam material is used to calculate the break-induced flow impact on the longitudinal and transversal expansion of the break;the break slope stability analysis is made to simulate the later intermittent expansion of break caused by sudden dam collapse;the balance between the flow discharge through downstream notching and that through dam top break is comsidered to establish their mutual relationships. Finally, the model is used to calculate the process of break development and the discharge hydrograph of an earth-rock dam due to overtopping failure. The result is in good agreement with the observed data.