Abstract: In order to study the critical condition of rainfall-induced saturated-zone forming， the soil saturation based on the MVG model was used to characterize the calculation equations of rainfall intensity. When the surface saturation of the soil reaches above 0.9 for the first time， the saturation is continuously stable at 0.9 ± 0.005 as the critical criterion of the saturated-zone. The numerical calculation method was used to obtain the critical rainfall intensity and rainfall time in the saturated-zone of the soil column and slope. Through the data fitting， a rainfall intensity-time critical curve model of the saturated zone was constructed， and the influence of different slope and effective rainfall intensity on the critical curve was analyzed. The results showed that the calculation method of saturation characterizing rainfall intensity can make the soil surface saturation reach a predetermined value， with an average accuracy of -0.008. When the effective rainfall intensity is large， the greater the slope， the shorter the time consuming to produce the saturated zone for the slope of 50°~60°，on the contrary， the longer it takes. A slope of 30°~40° is a “dangerous slope” prone to shallow landslides.

Key words: rainfall infiltration, saturated-zone, slope angle, critical rainfall intensity, Mualem-van Genuchten model