To address the persistent challenge of balancing magnetic permeability and core loss in soft magnetic composites (SMCs) fabricated from conventional spherical particles for high-frequency applications, this study proposes an innovative strategy using flaky FeSiAl particles as the base material. A novel FeSiAl/Al2O3 composite is synthesized by integrating interfacial chemical reactions with magnetic field-assisted orientation. A dense and uniform Al2O3 insulation layer is formed in situ via the reaction between the FeSiAl particle surface and a NaOH-based system. Subsequently, the application of an external magnetic field facilitates the alignment of flaky particles along the easy magnetization axis, thereby constructing a layered insulating architecture. The results demonstrate that the particle aspect ratio plays a crucial role in determining the morphology, thickness uniformity, and adhesion quality of the Al2O3 layer, which in turn influences the magnetic properties of the composites. Particularly, the sample with an aspect ratio of 141 achieves a highly continuous insulation layer, exhibiting an excellent combination of effective permeability (126.3), low power loss (108.0 kW/m3), and high electrical resistivity (331.1 Ω?m). Density functional theory (DFT) simulations reveal that strong covalent bonding at the FeSiAl/Al2O3 interface significantly enhance interfacial stability and reduces charge carrier mobility. Furthermore, a three-dimensional separation model based on Bertotti's loss theory is applied to quantify hysteresis, eddy current, and excess losses under varying particle geometries. The study provides in-depth insights into the structure-property relationship by correlating particle shape with magnetic behavior, confirming that optimized aspect ratio engineering can effectively suppress magnetic dilution and enhance energy efficiency. This study not only reveals the critical influence of the aspect ratio of flaky particles on the regulation of microstructure and the optimization of electromagnetic performance, but also establishes a solid theoretical foundation and a feasible technological pathway for the design and fabrication of high-frequency, low-loss soft magnetic composites, demonstrating significant potential for practical engineering applications in advanced electromagnetic systems and energy-efficient devices.