Abstract: Rainfall-induced landslides represent a frequent geological hazard in loess regions, with their triggering mechanisms closely linked to slope gradient. Using an indoor flume model test system, four homogeneous loess slope models with gradients of 25°, 30°, 35°, and 40°were constructed to study rainfall infiltration processes and instability mechanisms under different slope angles. The results indicate that: (1)Slope gradient significantly influences rainfall infiltration response: steeper slopes exhibit more pronounced rainwater splash erosion, clearer deformation at the slope shoulder, and relatively delayed surface erosion damage. The deformation and failure process can be divided into four stages: surface erosion and toe failure, gully development and expansion, creep sliding and local failure, and overall instability. (2)Under continuous rainfall, pore water pressure and earth pressure inside slopes of different gradients show noticeable fluctuations corresponding to slope erosion, crack development, and local deformation. (3)When the slope angle exceeds 30°, the wetting front migration rate gradually decreases with increasing slope steepness. At 30°, the rainfall infiltration rate couples most effectively with the soil′s water retention capacity, and the shear strength decreases most rapidly, making landslide initiation most likely. (4)As slope angle increases, the critical volumetric water content at the rear of the slope also rises. Before failure, the critical volumetric water content for a 40°slope is approximately 19% higher than that for a 25°slope. (5)Landslide tilting deformation is mainly controlled by shear slip or rotational movement along the slip surface. Prior to failure, soil deformation decreases gradually from the surface inward. Shear sliding behavior in the middle-upper soil layers is most pronounced at steeper slopes, and the change in basal inclination angle during failure is negatively correlated with slope gradient.