A single-baffle metal-dielectric-metal (MDM) waveguide coupled disk cavity cascade structure is proposed based on the transmission characteristics and photon local characteristics of the surface-plasmon-based sub-wavelength structures. The discrete state provided by the side-coupled disk cavity cascade and is used to provide a continuous state produced by the baffle plate placed in metal-dielectric-metal waveguide interferes constructively or destructively, and thus two different modes of Fano resonance are formed. The transmission characteristics of the formed Fano resonance are analyzed according to the coupled mode theory. The structure is simulated by the finite element analysis method and the effects of the structural parameters on the refractive index sensing characteristics are quantitatively analyzed. The effects of temperature and humidity on the measurement results in the actual measurement process are analyzed, based on the physical mechanism underlying the refractive index change. Moreover, the problem of cross-sensitivity in the sensing process is effectively solved by the differential sensing method.