We design a dual-band absorber based on metal multiplexed gratings at mid-infrared (mid-IR) frequencies. The metal multiplexed gratings are realized with two constitutive gratings and separated from a metallic ground plate by a thick dielectric spacer in order to construct the metal–insulator–metal architecture. Simulations indicate that two different absorption frequencies from the excitation of surface plasmon–polariton modes can be achieved under normal incidence, due to the different field distributions of each resonance node numbers in supercell. The doublet resonance positions can be independently and arbitrarily adjusted by varying the constitutive grating periods. We also confirmed that the absorption performance of the resonance peak is related to the non-uniform distribution level of the corresponding constitutive grating ridge. The designed absorber can be easily fabricated with planar processing technology and used in many applications where multispectral control of mid-IR signals is required.