Abstract: Spinach products in China have constituted over 90% of global output in recent years. Among them, root-cutting into the soil has been the sole harvesting method, in order to fully meet the national needs of large-scale production. It is very necessary to reevaluate the existing interaction models among spinach roots, soil, and cutting implements during harvesting. Current root-soil complex models cannot consider the influence of fibrous roots on the mechanical properties during root cutting, leading to the imprecise parameters of calibration. In this study, a discrete element model was presented to simulate the actual growth of spinach. A comparative framework was then established to analyze the scenarios both with and without fibrous root representations. A systematic analysis was implemented on the external dimensions of spinach taproots, the growth angles, and the distribution characteristics of fibrous roots in the soil matrix. A complete soil model was established using the discrete element method, including taproots and fibrous roots. A series of tests were conducted, including taproot accumulation, shear testing of the taproot, tensile testing of fibrous roots, and soil accumulation. The intrinsic, contact, and governing parameters were calibrated to determine the interactions between spinach roots and the soil medium. Calibration results demonstrated that the relative errors of simulation and measured average values in the taproot repose angle and ultimate shear force were 0.23% and 0.98%, respectively. Similarly, the relative error for the ultimate tensile force of fibrous roots was recorded at 0.9%, while the soil repose angle exhibited a relative error of 0.18%. A discrete element model of the spinach root-soil complex was then established using calibrated parameters. A validation test was conducted for the ultimate shear force within this complex. Single-factor testing revealed that the ultimate shear force escalated with the increase in the static friction coefficient and the bond radius coefficient between the root and soil within the specified range of factor values. Conversely, there was an initial increase followed by a decrease in response to the rising values of the rolling friction coefficient, normal stiffness per unit area of the bond, critical normal stress, and the contact radius coefficient of soil particles. Notably, the shear stiffness per unit area of the bond between roots and soil particles shared a negligible influence on the ultimate shear force. While the critical tangential stress also exerted a minimal but non-negligible effect. Box-Behnken response surface optimization indicated that the significance of influencing factors on the ultimate shear force was ranked in the descending order of the critical normal stress of the bond, static friction coefficient between spinach roots and soil, bond radius coefficient, rolling friction coefficient between roots and soil, soil particle contact radius coefficient, and bond unit area normal stiffness. The relative error of ultimate shear force between the optimized and measured values was recorded as 0.44%, indicating the high accuracy and reliability of the spinach root-soil complex model. The mechanical characteristics were effectively captured in the root cutting during spinach harvesting. The comparative analysis was also made on the root cutting characteristics-contrasting scenarios with/without fibrous roots. It was found that the ultimate shear force of the root-soil complex without fibrous roots was approximately 11.4% lower than that of the complex inclusive of fibrous roots. Moreover, the relative displacement between the main root and the soil increased by roughly 60.3%. It infers that the fibrous roots were used to bind the soil and then stabilize the taproot. The accuracy of the simulation was also enhanced by the spinach root system model. These findings can also provide a strong reference to facilitate the interaction dynamics among spinach roots, soil, and cutting tools.