Abstract: Paddy soils in the hilly and mountainous areas of southern China were soft and had low bearing capacity, conventional rubber-wheeled and crawler-type traveling components were therefore prone to sinkage and slippage during field operations. Paddy impellers generated driving force by inserting their blades into the soil and effectively reduced sinkage and slippage problems. After rice harvest, the remaining rice straw and roots adhered to the soil formed the rice straw-soil composite. During subsequent field operations, the composite impeded impeller rotation and increased traveling resistance. Because field conditions made the interaction parameters between the paddy impeller and the composite difficult to measure directly, physical experiments and DEM (discrete element method) simulations were combined to calibrate and validate a numerical model for structural optimization and performance prediction of paddy impellers. To address this issue, the basic physical and contact parameters of rice straw and soil were first measured, the geometric shape of the rice straw was obtained by three-dimensional scanning and reverse modeling, and the resulting particle-based straw models were embedded in a soil particle bed. The Hertz-Mindlin with Bonding and EEPA (edinburgh elasto-plastic adhesion) contact models were then used to represent bonding, elastoplastic deformation, and adhesion within the composite. Physical three-point bending tests were conducted on the rice straw, and the measured maximum bearing force of 6.02 N was used as the calibration target, 13 Plackett-Burman simulation experiments identified the normal stiffness per unit area, tangential stiffness per unit area, and particle bonding radius of the rice straw as the 3 significant parameters. A steepest ascent experiment and 17 Box-Behnken response surface experiments were subsequently performed to optimize these parameters. The optimal normal stiffness per unit area, tangential stiffness per unit area, and particle bonding radius were 3.92×10⁸ N/m³, 1.45×10⁷ N/m³, and 2.55 mm, respectively. The three-point bending simulation with the optimal parameter combination produced a maximum bearing force of 5.89 N. The measured maximum bearing force was 6.02 N, and the relative error was 2.15%. Physical shear tests were then conducted on the rice straw-soil composite, mean shear force of 89.2 N at a shear displacement of 45 mm was selected as the calibration target. Among the nine candidate parameters, the rice straw-soil static friction coefficient, contact plasticity ratio, and stiffness per unit area were identified as significant. Steepest ascent and Box-Behnken response surface experiments were then conducted for parameter optimization. The optimal values of the rice straw-soil static friction coefficient, contact plasticity ratio, and stiffness per unit area were 0.235, 0.746, and 1.107×10⁷ N/m³, respectively. A shear simulation using parameter combination produced a force of 85.3 N, with a relative error of 4.37% compared with the measured force. The composite model was further validated through plate sinkage and soil compactness tests, for which the relative errors between simulated and measured results were 4.12% and 6.36%, respectively. Finally, coupled MBD-DEM (multi-body dynamics-discrete element method) simulations of the paddy impeller were conducted, and results were compared with those of bench tests. Under a horizontal load of 850 N, an impeller rotational speed of 28.6 r/min, and a blade penetration depth of 200 mm, the relative errors in torque and vertical force between the simulations and bench tests were 8.50% and 0.43%, respectively. When blade penetration depths of 100, 175, and 250 mm were tested at 28.6 r/min, both driving efficiency and vertical force increased as penetration depth increased. When rotational speeds of 19, 24, and 29 r/min were tested at a fixed penetration depth of 175 mm, driving efficiency decreased with increasing speed, whereas vertical force changed slightly. The simulated and experimental results showed consistent trends. These results supported the accuracy of the DEM parameters calibrated from the three-point bending and composite shear tests, the calibrated composite model provided a basis for predicting the interaction between paddy impellers and the rice straw-soil composite and offered a reference for the design optimization and operating performance prediction of soil-engaging components in paddy machinery, including boat-type tractors.