Fe73.5Cu1Nb3Si13.5B9 (or FeCuNbSiB) powder/ S-glass fiber-reinforced epoxy composite panels were prepared by mold pressing method. Metallographic analysis shows that the amorphous powders are evenly distributed between the layers of S-glass fibers. The effects of the FeCuNbSiB powder mass fraction on the complex permittivity, complex permeability, and microwave absorption of the composite panels have been studied in the frequency range of 2.6-18.0 GHz. The complex permittivity of the composite panels with different mass fractions of the FeCuNbSiB powders shows several peaks in the 2.6-18.0 GHz frequency range. The complex permeability of the composites decreases with the increasing frequency in the frequency range of 8-18 GHz. The composite with FeCuNbSiB/epoxy mass ratio of 2.5:1.0 has excellent microwave absorption properties of a minimum reflection loss value-30.5 dB at 10.93 GHz for a thickness of 2 mm. A reflection loss exceeding-10 dB can be obtained in a broad frequency range of 3.2-18.0 GHz with a thickness of 1.15-5.00 mm. For the FeCuNbSiB composites, the magnetic loss is the dominant term for microwave absorption. The FeCuNbSiB powders are a possible candidate for high-performance microwave absorption filler. Fe73.5Cu1Nb3Si13.5B9 (or FeCuNbSiB) powder / S-glass fiber-reinforced epoxy composite panels were prepared by mold pressing method. Metallographic analysis shows that the amorphous powders are evenly distributed between the layers of S-glass fibers. The effects of the FeCuNbSiB powder mass fraction on the complex permittivity, complex permeability, and microwave absorption of the composite panels have been studied in the frequency range of 2.6-18.0 GHz. The complex permittivity of the composite panels with different mass fractions of the FeCuNbSi B powders shows several peaks in the 2.6-18.0 GHz frequency range. The complex permeability of the composites decreases with the increasing frequency in the frequency range of 8-18 GHz. The composite with FeCuNbSiB / epoxy mass ratio of 2.5: 1.0 has excellent microwave absorption properties of a minimum reflection loss value-30.5 dB at 10.93 GHz for a thickness of 2 mm. A reflection loss exceeding-10 dB can be obtained in a broad frequency range of For the FeCuNbSiB composites, the magnetic loss is the dominant term for microwave absorption. The FeCuNbSiB powders are a possible candidate for high-performance microwave absorption filler. 3.2-18.0 GHz with a thickness of 1.15-5.00 mm.