In this paper, a sandwich composite with broad-band wave-absorbing properties in C-Band was designed and prepared , and the electromagnetic parameters and reflectivity of the composite board were analyzed by a coaxial cable method and a vector network analyzer. The thickness of the composite laminate is 5 mm, and the surface layer of the board and the bottom layer are the glass fiber/ epoxy resin composite material, and the Fe50Ni50 powders/ butyl rubber nanocomposite is the interlayer. A spherical Fe50Ni50 powders with a particle size of about 100 nm were prepared by a liquid-phase reduction method, and a two-step blending method was used to prepare the Fe50Ni50/ butyl rubber nanocomposite. The result shows that the wave-absorbed mechanism of the interlayer is the magnetic loss in the frequency band of 2-18 GHz . The matching of the surface layer and the interlayer is the key to obtain the broad-band wave-absorbing properties, and the good wave-absorbing properties can be obtained by adjusting the thickness of the surface layer and the interlayer. When the thickness of the composite laminate is 5 mm and the thickness of the interlayer is 2 mm, the wave-absorbing frequency band for R being less than -10 dB, is in the range of5.6 GHz -7.6 GHz and 16.8 GHz -18 GHz, and the width of wave-absorbing band reaches 3.2 GHz, and the wave-absorbing bandwidth in the C-band is 2 GHz, which has achieved a breakthrough.