Abstract: Rainfall-induced landslides are among the most frequent and destructive geological hazards in the loess regions of China. To investigate the deformation process and instability mechanisms of loess slopes under rainfall conditions, a series of indoor loess slope rainfall tests were conducted using a self-designed rainfall simulation platform. Nine test conditions were established by combining three rainfall intensities (20 mm/h, 40 mm/h, 60 mm/h) and three slope angles (30°, 40°, 50°). Real-time monitoring was performed on soil moisture content, pore water pressure, and slope surface morphology. By comparing the deformation and failure characteristics under different working conditions, the failure patterns and instability mechanisms of loess slopes under different rainfall intensities and slopes were elucidated, providing theoretical guidance for the prevention and mitigation of rainfall-induced loess landslides. The results indicate that: (1) The increase in slope weakens the efficiency of rainfall infiltration and replenishment, but significantly accelerates the mechanical degradation of potential slip surfaces. The presence of preferential fracture networks facilitates rapid seepage, causing steeper slopes to reach critical instability under shorter cumulative rainfall durations. (2) Higher rainfall intensities lead to more severe slope erosion and localized deformation, faster infiltration rates, and more pronounced responses in moisture content and pore water pressure. Consequently, less rainfall is required to trigger slope failure. (3) Under the same cumulative rainfall conditions, different rainfall patterns result in distinct landslide failure modes. Long-duration, low-intensity rainfall tends to induce "overall traction-type landslides" on steep slopes and "localized collapses" on gentle slopes. In contrast, short-duration, high-intensity rainfall is more likely to trigger "shallow slide-flow" failures on steep slopes and "shallow flow slides" on gentler slopes. (4) Accelerated displacement has a predictive effect on landslide instability and failure. The accelerated growth process of pore water pressure is basically synchronized with the accelerated development process of slope deformation. Analyzing the spatial distribution of slope deformation, deformation rates, and pore water pressure variation can serve as critical indicators for early identification of loess landslides. (5) Targeted mitigation strategies are proposed: for high and steep slopes, a stepped and segmented slope design is recommended; for shallow slides on gentle to moderate slopes caused by intense short-term heavy rainfall, ecological-engineering composite reinforcement is adopted; for slopes that have already suffered local failure, a combined approach of toe back pressure filling and crest unloading is suggested.