Flaky Fe70Co12Si18 (atomic percentage) magnetic powders were prepared by quenching fastly combined with melt-spinning and high energy ball milling, followed by phosphorization with phosphoric acid alcohol solution. Effects of the concentration of the phosphoric acid solution on both the electromagnetic parameters and the microwave absorption performance of the FeCoSi powders was disclosed. With increased phosphoric acid concentration, both the real and imaginary parts of the permittivity decreased, while the real part of the permeability experienced subtle change with slight decrement for the imaginary part of the permeability. What’s more, enhanced reflection loss (RL) has been achieved with increased phosphoric acid concentration. The sample after 20%（mass fraction） phosporization exhibited optimal absorption with a minimum RL of -33.4 dB and a broad effective bandwidth of 5.2 GHz at a small thickness of 1.6 mm. The magnetic powder exhibited large natural resonance frequency due to the enhanced shape anisotropy after ball milling. The natural resonance frequency further improved with increased phosphoric acid concentration. The phosphate coatings fabricated on the surface of the powders after phosphorization not only effectively improved the impedance matching by increasing the resistivity and adjusting the permittivity of the material, but also enhanced the interface polarization by introducing interfaces between the powers and the phosphate layers.