China is one of the countries with the most serious disasters of dammed lakes, where the dammed lakes pose a great threat to the safety of people’s lives and properties among the upstream inundated area and the downstream outburst flood routing area. A deep understanding of the scouring and breaching processes of landslide dam can provide important scientific and technological basis for the design of emergency discharge channel and the formulation of downstream emergency measures. According to the event of 2018 Baige landslide-dammed lake in the Jinsha River, the scouring and breaching processes of landslide dam were systematically studied by conducting the laboratory physical modeling tests. Test results showed that the erosion and breaching processes of landslide dam could be divided into four stages, i.e., flow incubation stage, retrogressive scouring stage, dam break development stage, and riverbed rebalancing stage. When the retrogressive erosion scarp was traced back to the upstream slope top and the inlet section of the discharge channel was eroded, the discharge channel would connect to form a ramp with the bottom slope i>0. Then the flow velocity and flow rate increase suddenly, and the dam break developed rapidly. The results also showed that when the excavation width of the chute remained constant and the depth increased, the peak flow would decrease, the peak time would be delayed and the process of the burst flow became smoother; when the depth of the chute was fixed, the peak time would be delayed as the width increased. Finally, according to the testing results, some suggestions were put forward for the optimal design of the discharge channel: The location of the discharge channel should be arranged in the pass with the lowest crest elevation to reduce the peak flow and shorten the breaching time; when excavating the spillway, priority should be given to increasing its depth to minimize the water level of the dammed lake during dam breaching.