To investigate the dynamic process of tailings pond overtopping failure and the mechanism of breach development, three sets of comparable model tests were conducted to systematically analyze the process of breach spreading and the evolution of the breach. A power function formula of the breach spreading rate under overtopping failure mode was proposed. The process of infiltration line change during the reservoir water level rise stage and the change of flow velocity, flow rate, and breach width under different infiltration line depth conditions during the breaching stage was discussed. The results show that the breaching process of tailings dams can be subdivided into three stages, which are diffuse flow and gully formation, dam collapse and rapid expansion of the breach, and stable flow release. The depth of the infiltration line has a major impact on the flow rate, flow velocity, and the final width of the breach. The peak flow rate and the final width of the breach when the seepage fails are 1.5 times those of the normal discharge, and the peak flow velocity can be 1.2 to 1.3 times that of the normal discharge. Reducing the height of the infiltration line can delay the time for the debris flow reaching the downstream of the breached dam, which can extend the emergency evacuation time by about 3 min. The calculated values of the power function formula match the measured values well.