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With the aim to reduce the resistance during root-cutting operation, the physical properties and root distribution characteristics of grasslands with different degradation degrees were analyzed by measuring the soil firmness, bulk density, water content, porosity, root content, and root diameter. The constitutive relationship of the grassland soil-root composite was studied through triaxial tests of the undisturbed soil of the grassland and the remolded soil. The constitutive relationship of grassland soil with different degradation degrees was verified by numerical simulation. It was found that the physical characteristics and root distribution characteristics of grasslands with different degradation degrees were quite different, and the physical condition of degraded grassland with coverage of 30%~50% was the best. The cohesion, shear strength, elastic modulus, and secant modulus of undisturbed grassland soil samples were decreased with the aggravation of grassland degradation, and the shear strength of root-cutters on grassland should be greater than 24303kPa. The relative error of the deviator stress limit value obtained by the simulated triaxial test and the actual triaxial test was less than 8%, and the triaxial test of grassland soil simulated by ABAQUS was reliable. The constitutive parameters can be directly applied to the numerical modeling of grassland soil. The research can provide a theoretical basis for grassland root-cutting operation and the optimal design of root cutter.