In order to realize the requirements of small structure and high sensitivity of optical refractive index sensor, a single baffle metal-insulator-metal (MIM) waveguide coupled with cloud like cavity is proposed according to the transmission characteristics of surface plasmons. This structure refers to the concept of "cavity within a cavity". Under the action of near-field coupling, the wide continuous state formed by a cloud like cavity and the narrow discrete state formed by a metal baffle can be eliminated by interference, resulting in three Fano resonances with different modes. Combined with the coupling mode theory, the generation mechanism of triple Fano resonance is analyzed, and the finite element analysis is used to simulate the structure, and the influence of different structural parameters on the refractive index sensor characteristics and quality factors is quantitatively analyzed. The results show that the sensitivity of the three resonant modes are 600, 800 and 1083 nm/RIU, and the high quality factors are 5.08×10 ⁴, 3.56×10 ⁵, and 1.17×10 ³, respectively.