In this paper, we design a plasmonic absorber with a three-layer structure in the mid-infrared region. The bottom metal layer is a silver metal square layer, the middle layer is a silicon dioxide dielectric layer, and the top metal layer is silver nanoring arrays. When the incident light is perpendicular to the absorber surface, the reflection, transmission and absorption spectra of different size element arrays are numerically simulated with the three-dimensional finite element method. It is shown that a perfect absorption peak is achieved at a wavelength of 2900 nm, with a half-height width of nm and an absorptivity of. The tunable range of resonance wavelength is 400 nm by varying the dimensions of this structure.