<p> Periodic water fluctuations in the reservoir weakens the slope stability during the operation cycles, inducing significant but usually underestimated temporal variability in addition to inherent spatial variability of soil shear strength parameters in sliding zone. Herein, the Huangtupo No.1 landslide in the Three Gorges Reservoir Area serves as a study case. Initially, geostatistical methods were employed to investigate spatial variability of shear strength parameters. Subsequently, the consolidated drained (CD) triaxial tests were conducted on the deep slip zone soils to explore the temporal weakening laws under wetting-drying cycles. Furthermore, numerical simulation tests considering spatiotemporal variability of shear strength parameters in the saturated regions were designed using Monte Carlo and system reliability analysis methods to assess the stabilized slopes with double sliding zones, with discussion to individual impacts of spatial and temporal variability on stability. The test results reveal that the shear strength parameters of the deep slip zone soils follow a negative exponential weakening trend under wetting-drying cycles. Numerical simulations indicate a strong correlation between the stabilized slopes and reservoir water fluctuations, with the failure probability (FP) of the deep sliding zone exhibiting a more pronounced response to the weakening effect. Moreover, the temporal weakening effect induced by water fluctuations on the slip zone soils surpasses the spatial variability of shear strength parameters. This work may offer a contribution to risk control in similar reservoirs affected by ceaseless water fluctuations.</p>