A metal-dielectric-metal (MDM) waveguide-coupled square cavity structure with bimetallic baffle is proposed based on the transmission characteristics of surface plasmon polaritons in sub-wavelength structure. The Fano resonance is an asymmetric spectral line formed by the destructive interference between the wide continuous state generated by the Fabry-Perot (F-P) resonator and the narrow discrete state interference generated by the square cavity. Based on the coupled mode theory, the generation mechanism of the Fano resonance of the structure is qualitatively analyzed. The structure is simulated by finite element method , and the influence of structural parameters on the refractive index sensing characteristics is qualitatively analyzed. The results show that the refractive index sensitivity of the proposed structure reaches 1080 nm/RIU and the figure of merit reaches 7.35×10 5 after optimizing the parameters.