In order to achieve highly sensitive and reliable sensing detection of refractive index, according to the characteristics of Fano resonance sensing, a metal-medium-metal (MIM) waveguide coupled rectangular cavity structure is proposed in this paper to achieve high performance detection of Fano resonance. Based on the theoretical analysis of the coupling mode and the numerical simulation by the finite element method, the Fano transmission spectral characteristics of the rectangular cavity and its embedded metal plate are first compared and analyzed, and then the parameters of the designed rectangular cavity embedded metal plate structure are optimized and its sensing performance is analyzed. Two Fano mode resonant peaks are formed after incident light is incident in the waveguide in TM mode into the rectangular resonant cavity. The first mode has FOM of 9.4104 and a sensitivity of 700nm/RIU, while the second mode has FOM of 8.4103 and a sensitivity of 1200 nm/RIU. This indicates that the structure is designed to achieve dual Fano peak detection with high quality factors for each mode, which provides a theoretical basis for the design of a high-performance micro and nano optical sensor.