In order to achieve highly sensitive and reliable sensing detection of refractive index, a metal-insulator-metal (MIM) waveguide coupled rectangular cavity structure is proposed based on sensing performance of Fano resonance in this paper. With the help of theoretical analysis of the coupling mode and the numerical simulation by the finite element method, the Fano transmission spectral characteristics of the rectangular cavity with and without embedded metal plate are analyzed. And the parameters of the designed rectangular cavity embedded metal plate structure are optimized further. Finally, the influence of structural parameters on its sensing characteristics is discussed. The results show that two Fano mode resonant peaks are formed after light is incident into the rectangular resonant cavity with TM mode. In the first mode, the FOM is 9.4×104 and the sensitivity is 700nm/RIU; in the second mode, the FOM is 8.4×103 and the sensitivity is 1200nm/RIU. The results indicate that bimodal detection can be achieved in this structure and a high quality factor can be achieved also in each model, which provides a theoretical basis for the design of high performance micro/nano optical refractive index sensor.