We propose a single baffle metal-dielectric-metal (MDM) waveguide coupled disk cavity structure based on the transmission characteristics and photon local characteristics of the surface plasmonic sub-wavelength structures. Two discrete states are provided by the disk cavity, and a metal baffle is used to produce a continuous state. The discrete states coupled with the continuous state lead to two different modes of Fano resonance. Then, the transmission characteristics of the Fano resonance are analyzed with the coupled mode theory, and the finite element analysis is used to simulate. Moreover, the effects of the structural parameters on the refractive index sensing characteristics are quantitatively analyzed. The results show that, at the first mode, the figure of merit (FOM) and the refractive index sensitivity of the optimized structure can reach to 1.7×10 5 and 710 nm/RIU, respectively. For the second mode, they can reach to 1.36×10 5 and 1105 nm/RIU, respectively. The structure can provide a theoretical reference for solving the problem of cross-sensitivity of the sensor in refractive index measurement.