In this paper, a design with high sensitivity of a plasmonic biosensor by waveguide system is proposed, based on Metal-Insulator-Metal (MIM) coupled with unique rectangular cavities, this structure numerically simulated using the Finite-Difference Time-Domain method (FDTD) in two Dimensions (2D), and analyzed for the optimal sensor performance, by detecting the resonance wavelength and varying the refractive index (RI). The results show two sharp transmission peaks with high transmittance and asymmetrical line-shaped Fano resonances achieved with high value of sensitivity is 3010nm/RIU, by taking the wavelength resolution reach as high as 3.84×10-6 RIU. Considering the standards of Chip-scale integrated planar photonic sensing, the newly designed of the proposed structure with such high sensitivity provides remarkable properties suitable for biosensors, filter, and provide a new possibility for designing compact and high-performance plasmonic biosensors devices.