In recent years, the continuous advancement of engineering construction in China's Loess Plateau region has resulted in numerous high-fill and deep-excavation loess-red layer composite slopes (HDLS). Rainfall infiltration, a primary factor influencing slope stability, has markedly increased the likelihood of landslide disasters. To investigate the primary sliding failure mechanism of HDLS, a series of physical model tests and numerical simulation analyses were conducted on a loess-red layer composite slope in Zhongliang Town, Tianshui City. The study systematically examined the infiltration characteristics, deformation response, crack extension, and failure patterns of HDLS under rainfall conditions. The results demonstrate that (1) analysis of monitoring data from multi-sensors and real-time slope imagery revealed that under constant rainfall intensity, the slope failure process accelerates significantly with increasing slope height and angle. Furthermore, the slope shoulder exhibits greater susceptibility to large-scale sliding deformation due to enhanced rainwater infiltration. (2) Numerical simulations using Geo-studio reveal that the slope safety factor decreases significantly under fill and excavation conditions. The safety factor of high-fill slopes exhibits a slower decline with prolonged rainfall duration. (3) Rainfall erosion readily develops gully sliding ways on fill slopes, with tension cracks in the mid-slope region serving as primary infiltration channels that constitute the dominant failure reason. As the excavated slope angle increases, both the slope toe and mid-slope experience significantly enhanced unit water flow, intensifying surface runoff erosion, and the failure pattern exhibits progressive upward development originating from the slope toe to the upper slope. This paper investigates the hydrologic response characteristics and sliding deformation mechanisms of HDLS under various working conditions. The findings provide a significant reference value for protecting such high and steep slopes.