[Objective] Biochar plays an important role in improving soil physicochemical properties and enhancing soil structural stability. It achieves this by increasing soil organic matter content, enhancing nutrient retention and availability, and improving soil structure changes that further promote soil aeration, water infiltration, and root penetration, thereby strengthening soil structural stability. Soil structural stability is critical for sustainable agricultural production, as it influences soil erosion resistance, water-holding capacity, and overall soil health. However, few studies have explored the effects of biochar addition on the mechanical stability of soil structure from the perspective of rheological methods. [Methods] To address this research gap, a two - year field experiment was conducted. Soil samples with different biochar addition rates (0%, 1%, 3%, 5%, and 10%) were tested for basic physicochemical properties. Additionally, amplitude sweep tests were used to measure soil shear strength and viscoelastic parameters under varying water contents (30%, 37.5%, 45%, and 60%), aiming to analyze how biochar addition levels affect the shear strength and viscoelastic properties of the soils. The amplitude sweep mode is a robust rheological technique that allows for the assessment of soil behavior under different deformation conditions, providing valuable insights into soil mechanical stability. [Results] The results showed that biochar addition significantly improved soil structural mechanical stability by enhancing soil basic physicochemical properties, such as increasing soil cation exchange capacity and organic carbon content, which in turn increased soil energy storage modulus and shear strength This effect was particularly pronounced at lower water contents ranging from 30% to 37.5%. At these water contents, the soil structure was more stable and better able to resist external forces. Furthermore, biochar addition also resulted in an increased range of the linear viscoelastic zone and higher soil yield stress, thereby improving its structural stability. Notably, at a 10% addition rate, the soil maintained strong stability even under high water content conditions. [Conclusion] Biochar addition enhances the mechanical stability of soil structure by improving soil shear strength and viscoelasticity. These findings clarify the mechanical mechanism underlying soil structure improvement via biochar application, and thus provide a scientific basis for formulating targeted soil structure optimization measures in the Loess Plateau region.