Abstract: The application of plant root systems for soil reinforcement is widely used in soil and water conservation; however, the mechanical mechanisms underlying root anchorage and pull-out resistance remain insufficiently understood. To investigate the anchorage mechanisms of roots in soil and to establish a quantitative mechanical characterization model for guiding ecological restoration engineering, this study focused on the typical herbaceous species <italic>Medicago sativa</italic> (alfalfa) in Loess Plateau. Single-plant pull-out tests were conducted on undisturbed root-soil composites of alfalfa cultivated for 40 days to identify root failure modes, analyze the pull-out process and root system characteristics, and develop a segmented pull-out failure model based on the bond-slip theory of reinforced concrete interfaces. Model parameters were calibrated through experiments and validated using independent samples. The results indicated that: (1) root failure could be categorized into three typical modes: single-peak slip mode, double-peak (or multi-peak) fluctuation mode, and abrupt fracture mode, with single-peak slip failure mode being the predominant mode of root pull-out resistance; (2) characteristic values such as maximum pullout force, maximum pullout stress, and equivalent elastic modulus exhibited positive correlations with root ground diameter, while the displacements corresponding to maximum stress and residual stress showed no significant correlation with root ground diameter; the shape parameter <italic>n</italic> reflected the ductility characteristics of the root-soil interface, with smaller n indicating greater ductility, and this study confirmed the existence of an optimal range for n; (3) the root pull-out mechanical model based on Popovics theory demonstrated good fitting performance, with 83.3% of independent samples achieving a goodness-of-fit <italic>R²</italic>≥ 0.6. This study effectively characterizes the full-process mechanical response of roots from elastic deformation and interface softening to final pull-out, providing a new theoretical framework for quantitatively analyzing root anchorage mechanisms and predicting their mechanical behavior.