In order to explore the dynamic instability mechanism and risk of unsaturated soil slopes under rainfall infiltration, the dynamic evolution patterns of pore water pressure and safety factors of slopes under different rainfall intensities are systematically analyzed by coupling the SEEP/W unsaturated seepage module with the SLOPE/W limit equilibrium method by means of GeoStudio software, and the Monte Carlo method is used to quantitatively assess the risk of instability. The results indicate that rainfall intensity leads to a significant increase of matric suction in the shallow unsaturated zone, and the enlarged pore water pressure gradient drives negative pore pressure to propagate deeper into the slope. The safety factor shows a time-varying response characterized by a "rapid decline followed by asymptotic stabilization". Probabilistic analysis of strength parameters (cohesion and internal friction angle) using the Monte Carlo method shows that the probability of slope instability significantly increases with higher rainfall intensity, it is 5.9% under heavy rain (50 mm/d), rises to 15.4% under rainstorm (100 mm/d), and reaches 34.6% under torrential rain (200 mm/d). This study indicates that extreme rainfall will significantly intensify the adverse effects of variability in soil parameters, leading to a substantial reduction in slope stability. These findings provide theoretical support for early warning and prevention of unsaturated soil slope hazards.