The evolution of microcharacteristics of the loess affected by the combination of the permeability and seepage erosion significantly affect the integrity and the stability of the loess slopes. To investigate the in-behind mechanics of the loess under the rainfall, the self-designed experimental framwork was adopted to carry out the laboratory, during which the scanning electron microscope (SEM) tests, laser particle size analysis, as well as the numerical simulation analysis were conducted in parallel.The experimental results showed that the unsaturated permeability of the loess exponentially increases with the increased water content. The migration of clay particles alters the microstructure of the soil, resulting the enhancement of the porosity and permeability, associated with a significant clay particles loss within the shallow layer. These loess accumulated in the deep layer transformed into a lubricating water film layer with strong lubricity, which reduces the overall stability of loess slopes. Furthermore, the changed permeability results from the conversion between micropores, small pores, mespores, and macropores. Because that the internal erosion forms a layer of clay particle aggregation at a certain depth, the formation of slip surfaces of landslides was further promoted. The research is of great significantance, which makes contribute to the in-depth understanding about the mechanism of rainfall impact on the microstructure and unsaturated permeability characteristics of the loess.