The extensive fill engineering slopes formed by major projects such as "managing the ditch and creating land" in the Loess Plateau region face severe challenges of rainfall-induced instability. Inspired by the self-stable structure of natural loess-paleosol sequences and the Nature-based Solutions (NbS) concept, the water-control structure for loess fill slopes was propose, which is a composite of erosion-resistant surface layer (modified cellulose-treated) and low-permeability layers (mimicking paleosol properties using lime-improved loess). Indoor soil mechanics tests (liquid/plastic limits, compaction, permeability, shear strength) and Scanning Electron Microscopy (SEM) analysis revealed that both modified materials significantly enhance soil strength and reduce permeability (e.g., lime treatment reduced saturated permeability by 95.18%). Artificial rainfall model experiments demonstrated that slopes with water—control structure exhibit delayed infiltration response (up to 1,565 minutes), reduced erosion volume (71.6% less than untreated slopes), and shifted failure modes from fluidized collapse to gradual shear-slip. Numerical simulations (GeoStudio) further optimized the low-permeability layer configuration, identifying a 3 m-thick, 2°-inclined layer as optimal for maximizing stability. This study reveals that the NbS structure effectively regulates rainfall infiltration and erosion processes, significantly reducing erosion volume and altering failure modes. Consequently, the NbS-based water-control structure provides a theoretical basis and key technical support for the prevention and control of instability in loess fill slopes.