Abstract: High-altitude rockslide–debris avalanches are widespread in extremely high mountain regions and represent one of the most severe geological hazards threatening mountain communities and critical infrastructure. Investigating the rheological behavior of large-scale granular materials during high-velocity motion provides key insights into the dynamic mechanisms of such flows from a mesoscopic perspective. In this study, a series of large-scale physical model experiments on high-speed granular flows were conducted using a self-developed remote-controlled experimental platform for high-elevation geological hazards, under varying granular size and flow volume conditions. The results show that high-speed granular flows commonly exhibit a dynamic coexistence of dense, dilute, ultra-dilute flow regimes. The formation and evolution of these multiphase states originate from granular collisions and velocity fluctuations induced by internal velocity-gradient heterogeneity within the granular system. Increasing granular size promotes a transition in shear behavior from localized to system-wide differential shearing, intensifies collisional excitation, and shifts the granular system toward dilute states. In contrast, increasing flow volume markedly enhances shear rates within localized basal shear zones, leading to more pronounced vertical differentiation of dense, dilute, and ultra-dilute regimes. Furthermore, a clear correspondence is identified between collision–shear-dominated stress transmission mechanisms and the observed flow regimes. These findings indicate that dynamic modeling of high-elevation rockslide–debris flows should incorporate state variables characterizing granular concentration, granular temperature, and regime transitions to effectively capture multiphase structures and cross-regime evolution.