A multiband metamaterial absorber with typical sandwich structure in microwave region is proposed. The top layer is a square array with unit-cell of standing upright metallic concentric circular multi-ring structure, the middle layer is a dielectric slab, and the bottom layer is a continuous metallic slab. Using the fullwave simulation method, the absorption spectra of the metamaterial for different numbers of the rings in the unit-cell of the top layer are simulated. The results show that n absorption peaks appear when the unit-cell of the top layer owns n metallic rings. Detailed studies exhibit that the highest frequency absorption peak is related to the excitation of the electric dipole mode of the whole multi-ring unit-cell structure in the top layer, while other absorption peaks with lower frequencies are related to the excitation of the trapped-modes localized in the two corresponding neighbouring rings.