A novel symmetric plasmonic structure consisting of a metal-insulator-metal waveguide and a rectangular cavity is proposed to investigate Fano resonance performance by adjusting the size of the structure.The Fano resonance originates from the interference between a local quadrupolar and a broad spectral line in the rectangular cavity.The tuning of the Fano profile is realized by changing the size of the rectangular cavity.The nanostructure is expected to work as an excellent plasmonic sensor with a high sensitivity of about 530nm/RIU and a figure of merit of about 650.