Abstract: The morphology and phase structure of Sm FeN alloy powder before and after ball milling for 3 h, 6 h and 9 h,its electromagnetic parameters（ε', ε'', μ', μ''） and electromagnetic reflectivity RL values in the frequency range of electromagnetic waves of 2-18 GHz were systematically studied, and the electromagnetic loss mechanisms were discussed.The results show that ball milling has no effect on the phase composition of the powder, the particle size of the powder decreases with the extension of the ball milling time, the ball milling can affect the optical frequency dielectric parameters of Sm FeN to lead to a decrease of the real part of permittivity, and then increasing the impedance value, with the progress of the ball milling, the real part of the permittivity rises, the impedance value decreases, and there is a conversion from dielectricity to magnetism at 13-15 GHz, at which the real part of permittivity drops and in the meantime permeability increases sharply so that the impedance value decreases in this wave band. Before ball milling, the electromagnetic loss mechanism of Sm FeN powder is dielectric relaxation polarization loss and weak natural resonance loss. The two loss mechanisms are enhanced after ball milling, and the frequencies for the loss are changed, and the dielectric loss is still dominant after ball milling. Ball milling improves electromagnetic matching of the sample at 12-15 GHz, so that the sample ball milling for 9 h, at a frequency of 13.3 GHz, achieves the best reflectivity of 19.04 dB.