<p>Similarity theory provides a robust foundation for large-scale physical modeling of slope instability mechanisms associated with underground mining. Dynamic disturbances are a major contributing factor to slope failure, while infiltration-induced erosion and precipitation-related softening further compromise slope stability. This study utilizes similarity principles to investigate the coupled effects of rainfall and mining-induced stress variations in a mining site located in Jiangxi Province, China. A novel composite test apparatus was developed to simulate the combined effects of heavy rainfall and underground mining operations. Using this device, a scaled physical model of an open-pit to underground transition slope was constructed and subjected to controlled rainfall scenarios. The stability of the overburden rock was analyzed across different mining stages, revealing that the destabilizing effect of rainfall is significantly impacted by the phase of mining activity. Temporal stress variations were recorded and correlated with observed physical failure patterns in high-steep slopes. The results identify a three-stage process of stress-induced deformation under coupled rainfall and mining conditions: (1) initial gully formation, (2) crack propagation and slip surface development, and (3) overall instability culminating in large-scale sliding failure. Intense rainfall triggered surface runoff and gully erosion, promoting tensile crack formation, reducing rock mass strength, and expediting slip surface evolution. Concurrently, mining operations disrupted the internal stress balance, leading to progressive deformation and tensile failure, thereby increasing the risk of collapse. The failure initiated at the base of the open-pit slope and propagated upward, transitioning from localized instability to widespread structural failure characterized by fully developed slip surfaces.</p>