Rainfall-induced landslides are the most common geological hazard in the world, and the effect of preferential flow infiltration has been neglected for a long time regarding the slope reliability analysis under rainfall infiltration. The preferential infiltration under rainfall conditions is solved numerically by Comsol Multiphysics, and the slope factor of safety is calculated using an infinite slope model. An improved Cholesky decomposition method is applied to generate spatially correlated random fields, and the slope reliability during rainfall is analyzed using the Monte Carlo method. Combining the deterministic and reliability results to compare the variation of slope factor of safety between the homogeneous infiltration and the preferential infiltration during rainfall, it can be found that: (1) the preferential infiltration model is safer when the rainfall intensity is low, while the homogeneous infiltration is more stable at higher rainfall intensity; (2) the spatial variability of infiltration parameters is the crucial factor of slope failure in homogeneous infiltration, while the slope instability of preferential infiltration is mainly caused by the rapid advance of the wetting front; (3) for the study of preferential infiltration model, the slope has a higher probability of failure when the hydraulic exchange intensity between the matrix domain and the preferential domain, while smaller hydraulic exchange intensity may affect the slope failure at the bottom.