[Objective] Iron oxide morphology is a key factor controlling the mechanical behavior of red soils, but the role of amorphous iron oxide in shear strength remains poorly understood.[Methods] We investigated kaolinite, the dominant clay mineral in soils from a typical benggang area of Fujian Province, by adding amorphous iron oxide at different concentrations (1, 2, 3, and 10 g·kg?1). X-ray diffraction (XRD), scanning electron microscopy (SEM), and direct shear tests were used to evaluate its effects on soil microstructure and shear strength.[Results] XRD analysis revealed that amorphous iron oxide did not undergo a chemical reaction with kaolinite but primarily altered the mineral characteristics through physical coating. The concentration of amorphous iron oxide exerted a pronounced effect on the distribution of cemented aggregates across different particle sizes, demonstrating a significant regulatory role in the formation of aggregates within the 2–250 μm range. Among the microstructural parameters, only the particle long-axis and roundness exhibited significant changes: the long-axis decreased by 22% in samples with 3 g·kg?1 treatment, while roundness was reduced by 25% in samples with 2 g·kg?1 treatment. The coefficients of variation for all other parameters remained below 15%. The treatment with 3 g·kg?1 consistently exhibited the highest shear strength across all confining pressure levels. The enhancement in shear strength was closely associated with the observed microstructural alterations. Further analysis indicated that the 3 g·kg?1 treatment resulted in the most substantial improvement in cohesion, which increased by 24.82% compared to pure kaolinite. In contrast, the internal friction angle varied by less than 5.30%.[Conclusion] Amorphous iron oxide enhances soil shear strength by improving structure through cementation and encapsulation. These results provide theoretical support for understanding soil stability in benggang areas.