Abstract: New composite sandwich structures filled with foam concrete were proposed to improve the failure mode subjected to compression and enhance the energy absorption capacity. The composite structures were made by filling foam concrete of different densities (409 kg/m³, 575 kg/m³, 848 kg/m³, 1 014 kg/m³) into negative Poisson's ratio aluminum structures. The failure modes and energy absorption capacities of the specimens subjected to quasi-static compression were compared among the empty auxetic structures, the auxetic structures filled with foam concrete as well as the standalone foam concrete control specimens. Based on the load-displacement relationship and failure mode, the findings can be reached: With foam concrete of relatively low density filled, the Poisson’s ratio of the auxetic composite structures is limited to a small value, leading to auxetic deformation of the cells and progressive compression mode of the whole structures. With foam concrete of relatively high density filled, the structures exhibit X-type shear failure, in which the cell walls around plastic hinges and shear bands fracture. The energy absorption increases, and the densification strain of the composite structures decreases, with increasing density of the foam concrete. However, with filler density greater than a certain value, the enhancement of energy absorption of the composite structures becomes less remarkable, and the specific energy absorption decreases, with increasing density of the filled foam concrete.