<p>Rainfall is the most common natural factor that induces the deformation and failure of loess slopes, which is essentially a soil–water-air three-phase coupling process of water infiltration, pore-air migration, and soil deformation. At present, for the numerical study of unsaturated slopes, the setting of pressure boundary and initial value of the seepage field depends on the groundwater level distribution, however, many cases show that the occurrence of landslide is not related to the groundwater, this method cannot truly reflect the pore water pressure state of the soil. In this study, an on-site landslide was taken as an example, the equilibrium of the initial water field was considered when the slope was not affected by groundwater. The water field of the slope after the equilibrium of capillary pressure and gravity field was assigned to the calculation model, by which the influence of rainfall on the stability of the loess slope was explored. On this basis, combining macro and micro tests, the degradation of loess mechanical strength caused by wetting was analyzed, and a structural degradation model was established. The results show that the equilibrium of the initial water field solves the problem of setting the initial values and pressure boundaries for unsaturated seepage calculations. With the infiltration, pore air was compressed or displaced, which also blocked the downward migration of wetting front. Rainfall infiltration reduced the matric suction, and reduced the factor of safety. The test of loess revealed that water erosion caused the clay minerals attached to the surface of the particles to fall off, which weakened the cementation between the particles. The research results can provide new research ideas for the stability analysis of loess slopes less affected by groundwater in the Loess Plateau.</p>