The traditional storage methods for tailings and waste rock, characterized by large land occupation and significant environmental risks, hinder the sustainable economic development of mining operations. Therefore, there is an urgent need to explore a new disposal approach that combines both materials in a mixed stacking method. This study investigates the stability of solidified tailings and waste rock mixtures through large-scale indoor model experiments under different rainfall conditions. In the continuous 9-hour rainfall tests with intensities of 20 mm/h, 40 mm/h, and 60 mm/h, the changes in pore water pressure and soil pressure were monitored, and the evolution of parameters such as displacement fields and saturation was analyzed to reveal the microscopic deformations in the model tests. The results show that when the rainfall intensity reaches 60 mm/h, surface depressions and rill erosion on the slope significantly expand, but no sliding instability occurs, and overall stability remains good. The migration of fine particles due to rainfall infiltration increases the local density of the mixed stacking body, leading to a rapid increase in pore water pressure and soil pressure during the rainfall period. After rainfall ceases, the dissipation time of pore water pressure is prolonged, and the soil pressure exhibits a slight residual effect, slightly higher than the pre-infiltration values. These findings provide a new direction for the resource utilization of tailings and waste rock and their combined mixed stacking approach.