The stability of basal-ice moraine slopes (BIMS) is increasingly threatened by climate warming, posing a significant geohazard in high-altitude regions. The degradation of buried basal ice, coupled with intense rainfall, induces complex thermo-hydro-mechanical (THM) interactions, yet the specific failure mechanisms remain insufficiently understood. This study provides direct experimental evidence using instrumented laboratory flume experiments to investigate the failure characteristics of BIMS under controlled thermal and rainfall conditions. By systematically analyzing volumetric water content, soil pressure, pore water pressure and deformation, we clarify the regulatory role of the ice-soil interface in slope instability. Results demonstrate this interface is the primary control, governing heat transfer and meltwater redistribution. We identify two distinct failure pathways: temperature-driven instability is progressive, evolving from initial slope-toe failure to large-scale misaligned sliding, where rising temperatures accelerate meltwater accumulation. In stark contrast, rainfall-induced instability is abrupt, characterized by rapid slope disintegration and surface flow-slips, with intense rainfall showing the potential to trigger debris flows. These findings elucidate the coupled THM failure mechanisms of BIMS, providing a robust scientific basis for process-based hazard assessment and risk mitigation in cold-region mountainous environments.