Abstract: A series of Gully Reclamation Projects were carried out in Yan’an City to expand the area of available arable land. In these projects, contact interfaces formed between filled slope and original slope potentially influenced the seepage deformation and failure of the loess-filled slope. This paper aims to solve the deformation and failure of loess-filled slope in Yan’an City using the indoor rainfall model test. The influence of interface on seepage characteristics and deformation failure of loess-filled slope is analyzed, and the failure mechanism of loess-filled slope affected by the interface is revealed. The results show that: (1) the interface is a dominant seepage channel for rainwater infiltration in the loess-filled slope; along the seepage channel, rainwater infiltrates into the middle and bottom of the filling body and gathers in the middle of the filling body with the infiltration rainwater from slope surface; the process accelerates the saturation process of the filled slope, changes its seepage field and effective stress field, causes tensile stress at its top and shoulder, and induces the initiation and expansion of multiple tension cracks. (2) In the process, a small range of erosion occurs in the middle of the slope toe, and a large range of erosion damages in the middle of filled-slope surface; the erosion damage triggers a large-scale shallow sliding in the middle and bottom of the filled slope, and further intensifies the initiation and expansion of cracks at the top and shoulder of the filling body; finally, multiple dominant seepage channels and complex topographic feature are formed, and the local stability of loess-filled slope is gradually reduced. The research results are important for the stability evaluation of loess-filled slope and the service life of the Gully Reclamation Projects.