Abstract: To investigate the variation pattern of the phreatic line in a tailings reservoir under heavy rainfall conditions and the dynamic evolution process of tailings flow after dam failure, a tailings reservoir in Sichuan Province was taken as an engineering case. Key areas within its prototype range were selected, and a tailings dam failure simulation was established following similarity theory. A model with geometric dimensions of 3 m × 1 m × 1 m was employed. The variation pattern of the phreatic line under heavy rainfall and the evolution pattern of the released tailings flow after overtopping and dam failure were studied. The results show that the evolution process of tailings reservoir’s overtopping failure can be subdivided into 4 stages: overtopping initiation, vertical incision, lateral widening, and scour-siltation equilibrium. In the initial stage of overtopping and dam failure, the phenomenon of coexisting multiple initial breaches has a certain probability of occurrence. The formation and erosion degree of the main breach are closely related to the flow magnitude. The appearance time of the initial breach advances with increasing rainfall intensity; however, when the breach forms, the water level height in the phreatic line measurement tubes shows an opposite trend under different rainfall conditions, namely 80 mm/h > 100 mm/h > 120 mm/h. During the initial period of rainfall, the influence of rainfall amount on the phreatic line exhibits spatial variability. The phreatic lines at the far end and the middle section of the beach are significantly affected, while those in the middle and front sections of the dam body are essentially not noticeably disturbed by changes in rainfall amount. In other words, as it is closer to the reservoir tail, there is greater water infiltration, resulting in a significantly faster rise rate of the phreatic line in that area compared to near the dam body.