Fano resonances based on circle-with-inner-core and stub structure are investigated using data derived from the finite element method and theoretically explained by the multimode interference coupled mode theory and the electric distribution in the system. The parameters of specific structure are modified to investigate the influence of different parts for this metal–insulator–metal system. Due to the high sensitivity to filled dielectric materials, the proposed structure can be applied as a refractive index sensor, whose performances are also explored. High sensitivity is gained as high as 1183.3  nm  /  RIU. And the figure of merit, a key parameter to describe the sensing characteristic, is achieved as 5.1115  ×  10⁴, which is better than most similar structures. Our work is significant for the sensitive refractive index nanosensor.