Abstract: The rainfall-induced slope failure mechanism and reliability analyses rarely consider the spatial variability of hydraulic and shear strength parameters at the same time and ignore a fact that the slopes always keep stable under the natural condition. An infinite slope model is taken as an example to conduct probabilistic back analyses of spatially varying shear strength parameters using the observation information in advance. Then, a non-stationary random field model is established to simulate the spatial variability and non-stationary distribution feature of the hydraulic conductivity. The probabilities of slope failure and distributions of the critical slip surface under different rainfall durations are evaluated within the framework of Monte-Carlo simulation. Based on these, the rainfall-induced slope failure mechanisms considering the spatial variability of hydraulic and shear strength parameters simultaneously are investigated. The results indicate that the probability of slope failure evaluated based on the posterior information of shear strength parameters obtained from the probabilistic back analyses is reduced from 28.1% (prior) to 7.2%. It is found that the triggering factors for the slope instability are different for different rainfall stages. The rainfall-induced slope failure mechanism and probability of failure will be erroneously evaluated, especially at the initial stage of rainfall, if the field observation information is ignored.