In recent years, due to super-standard flood caused by heavy rainfall, the overtopping-induced dam breach events of small-sized reservoirs often occurred in China. The failure of Sheyuegou Reservoir in Xinjiang on July 31, 2018 is a typical case. Because detailed field investigation data is available, the back analysis of this case has important theoretical significance and engineering value. Based on the dam-break video and post-failure investigation, the dam breach process of Sheyuegou Reservoir has been clarified, and the erosion characteristics of dam-break flow has been summarized. A numerical calculation method for dam breach process has been established, and can consider the evolution rule of breach morphology in the longitudinal and cross sections. By comparing the numerical and measured results, it is found that the relative errors of important dam breach parameters (i.e., peak breach flow, final breach top and bottom widths, final breach depth, and time to peak) are all within ±15%. For a numerical model of dam breaching, the numerical results have high accuracy. The parameter sensitivity analysis shows that soil erodibility coefficient has an important effect on the dam breach process, especially on the peak breach flow and time to peak. The headcut migration coefficient also has a certain influence on the dam breach process, which determines the time to peak.