Abstract: Aiming at investigating the instability mode and dynamic response of the seepage field of the collapse wall under the combined interaction of groundwater uplift and different types of rainfall duration, on the basis of mechanical index parameters of rock and soil layers of the collapsing hill obtained from in-situ observation and relevant geotechnical physical model tests, we attempt to explore the process and mechanism of hydraulic factors affecting the collapsing gully erosion under a single rainfall from the angle of coupling seepage domain and stress field of collapse wall through performing plenty of numerical experiments. The FEM analysis results indicate that the failure mode of the collapse wall in successive light rain environment belongs to the combination of local erosion of the middle and lower soil layers of collapse wall and integral-sliding failure of the soil body above the sandy soil layer, and in short-term heavy rain environment, the failure mode of the collapse wall is manifested by the (layered) flow sliding of the shallow slope surface. Whereas, no matter what type of rainfall, there must be an earlier stage (lasting for a long time) of precipitation that causes the sandy soil layer of the collapse wall to be eroded by water to form a concave cavity. Until the depth of the concave cavity reaches an ultimate value, the volume of concave cavity will no longer expand, and it will turn into a later stage of precipitation before the collapse wall is in critical failure status. The seepage areas generated by rainfall infiltration mainly occur in the shallow soil layers of the collapse wall, as a result, the shear strength properties of the shallow soil unceasingly weaken and the maximum shear stress increases obviously. With the prolongation of rainfall duration, a transient saturated zone appears in the surface soil layer of the catchment slope and the wetting peak (zero pressure surface) gradually moves towards the deep strata of the collapsing gully wall. The groundwater level line gradually rises and thus causes pipe erosion to the bottom of the sandy soil layer by (downward) outcropping springs. The analysis results are in good agreement with the field observation phenomena.