In this study, a composite metamaterial structure comprising a gold-nanostructure top layer, an intermediate dielectric layer, and a metal base layer was proposed. An I-shaped cell array comprising three oval nanodisks is the top layer of the metamaterial nanostructure, silicon dioxide is the intermediate dielectric layer, and a gold film is the metal base layer. Herein, the absorption characteristics, electric field distribution, and refractive index sensitivity characteristics of the structure were studied via the finite element method. Thus, three absorption peaks can be observed, and the corresponding absorptions are observed to be 91.06%, 99.63%, and 97.26%. In addition, the influence of the structural parameters and surrounding media on the absorption and response characteristics with respect to the changes in the refractive index was studied. The maximum sensitivity is 425 nm/RIU (RIU is the refractive index unit), and the figure of merit is 14. This study would provide theoretical guidance for developing perfect absorbers by considering the surface plasmon metamaterial structure as a refractive index sensor.