Abstract: To reduce the high magnetic loss of FeSiAl powder-core inductors used in DC-DC boost circuits of unmanned aerial vehicle electronic speed controllers under high-frequency and high-current conditions, gas-atomized FeSiAl powders were insulated with a phosphoric acid-ethanol solution. The effect of phosphoric acid concentration on the high-frequency magnetic properties of the powder cores was systematically investigated. The morphology and structure of the coated powders were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. Effective permeability, DC-bias resistance, and high-frequency core loss were measured using an inductance-DC superposition measurement system and a soft magnetic dynamic measurement system. The results show that a relatively uniform phosphate insulating layer formed on the FeSiAl powder surface. As the phosphoric acid concentration increased, the insulating layer thickened, the volume fraction of the nonmagnetic phase increased, and the structural demagnetization effect was enhanced, leading to lower effective permeability but improved DC-bias performance. The high-frequency magnetic loss first decreased and then increased because of the competition between eddy-current suppression and hysteresis loss. At a phosphoric acid concentration of 1.0 wt%, the powder core exhibited the best overall performance, with an effective permeability of about 62 and a core loss as low as 91.9 mW·cm-3 at 100 kHz/50 mT. These results provide a useful reference for the design of high-frequency, high-power-density power inductors.