The invention relates to the field of wave-absorbing materials and particularly relates to a lightweight broadband structure wave-absorbing plate and a preparation method thereof. The lightweight broadband structure wave-absorbing plate comprises a honeycomb core material, a wave-absorbing coating, wave-transparent panels and a polyurea coating, wherein a honeycomb unit of the honeycomb core material is a hexagon; a solvent wave-absorbing coating is dipped on the wall of the honeycomb unit; the wave-transparent panels are laminated on an upper layer and a lower layer of the wave-absorbing honeycomb material after dip forming; and the polyurea coating is sprayed on the wave-transparent panel on the upper layer. Honeycombs are dipped by using the solvent wave-absorbing coating, and electromagnetic wave matching design is carried out on the wave-absorbing honeycombs, the panels and the polyurea coating, so that the wave-absorbing band is further broadened while the related environmental resistance is improved, and the wave-absorbing property is greatly improved.