Abstract: To investigate the spatiotemporal evolution of shear strength in waste-soil slopes under rainfall. Based on the spatial shear strength parameters of naturally stockpiled waste soil, this study developed a spatiotemporal shear strength model for waste-soil slopes under rainfall by establishing a strength equation that accounts for fines loss and clogging. The results show that: (1) for waste-soil slopes with the same gradation, the shear strength at the slope toe and within the slope mass decreases significantly, and some local zones exhibit pronounced abrupt changes in strength due to fine-particle clogging; in contrast, for waste-soil slopes with spatially heterogeneous gradation, strength variations are mainly concentrated near the slope crest, while the strength evolution within the slope mass remains relatively continuous. (2) Under long-term rainfall, the internal friction angles at the slope surface and inside the waste-soil slope with uniform gradation decrease by approximately 0.4°–2.7° and 1.4°–5.4°, respectively; whereas for the waste-soil slope with spatially heterogeneous gradation, the maximum reduction in the internal friction angle at the slope surface is about 3.4°, and changes within the slope mass are insignificant. (3) During rainfall, the waste-soil slope with spatially heterogeneous gradation exhibits intense migration of fine particles along the slope surface and their enrichment at depth, accompanied by dynamic adjustment of the pore structure. Fundamentally, this behavior is driven by continuous reorganization of the coarse-particle skeleton under the combined effects of seepage and surface runoff, resulting in a markedly spatially non-uniform evolution of shear strength. This study provides a basis for elucidating the stability evolution of waste-dump slopes during long-term dumping and storage.