Fano resonance with asymmetric and sharp spectral features has recently been intriguing for refractive index sensing. In this study, we demonstrate a Fano resonance sensor that employs the coupling between a metal-insulator-metal (MIM) waveguide and a semi-ring resonator. The MIM waveguide has a three-ring resonator built in the center, and high-field confinement is observed due to the coupling of the two structures. The coupled structure's transmission spectrum exhibits three Fano resonance modes that are influenced by structure geometry and the surrounding medium's refractive index. The high-quality factor ( ) of mode 3 indicates that this sensor is suitable for use in optical sensing applications. To achieve maximum sensing performance, the parameters of the proposed structure are manipulated and different sensing parameters are computed. The sensor's estimated sensitivity of 3164.97 nm/RIU is equivalent to that of other Fano resonance sensors. Additionally, for plasmonic MIM sensors, the developed sensor achieves high values of and of 5420.99 and 5641.57 , respectively. The proposed high-sensitivity sensor could be an attractive choice for sensing applications because of its straightforward design and ease of fabrication. Also, the combination of very high sensitivity and FOM in a tiny and compact configuration is ideal for on-chip plasmonic nanosensors