Abstract: High-thermal-conductivity Al₂O₃ insulating layers were directly constructed on the surface of FeSiAl magnetic powders via in-situ reaction with KOH solution, aiming to simultaneously realize low loss and efficient heat dissipation of soft magnetic composites. The regulation effect of reaction time (5~50  min) on the microstructure, insulation property, magnetic performance, and thermal management performance of the coating layers was systematically investigated. The results demonstrate that, the reaction time exerts the decisive influence on coating quality and final material performance. Insufficient reaction time leads to the incomplete coating and poor comprehensive properties, the excessive reaction time induces the loose coating structure and interfacial defects, which increases the hysteresis loss and aggravates phonon scattering. The continuous and dense Al₂O₃ coatings with favorable interfacial bonding can be obtained at the reaction time of 20~30  min, and the corresponding composites achieve the optimal comprehensive performance, the electrical resistivity reaches 487.2  Ω·m, the total core loss decreases to 141.5  kW·m⁻³ (@10  mT, 500  kHz), and the thermal conductivity increases to 18.2 W·m⁻¹·K⁻¹, which is 22.1% higher than that of the uncoated samples.