Abstract: Moraine dams are widely distributed in high and very high mountainous areas around the world. In order to investigate the breaching process of end-moraine dams and to understand the evolution of the breaches during the breaching process of end-moraine dams, this paper designed in situ experiments for simulating the process of moraine lake overtopping breakage with the Jialongcuo as the prototype. The experimental results show that: (1) According to the phenomenon of the breach process, the moraine dam breach process is divided into four stages: downstream slope scour, “headcut erosion”, undercutting of the outlet and widening of the breach. (2) The upstream lake collapse body excited by the surge will cause the instantaneous flow in the breach to increase several times, thus making the “headcut erosion” process under the conditions of surge and no surge in two forms of undercutting, which is steep hills and slopes. (3) By analyzing the undercut erosion process of the breach, it is found that the undercut erosion process of the breach is mainly influenced by the void ratio and fines content of the dam body, and there is a linear relationship between the erosion rate at the midpoint of the breach and the water flow shear stress, which is consistent with the linear erosion model. Through the analysis found that the erodible coefficient of Jialongcuo end-moraine dam is 0.051, the critical starting stress is 237.64 Pa, compared with the landslide dams, the erodible coefficient is smaller, while the critical shear stress is larger.