Based on the theory of Surface plasmon polaritons, a plasmonic structure consisting of a metal-insulator-metal waveguide and a semi-ring stub was proposed to investigate the transmission properties by the finite-difference time-domain method. Simulation results show that there is a sharp and asymmetric Fano profile in transmission spectrum, which originates from the interference between a discrete state and a continuous state in the semi-ring stub. The tuning of the Fano profile is realized by changing the size of the semi-ring stub, and the sensitivity and figure of merit of this structure is 575 nm/RIU and 5 671. Moreover, multiple fano resonances are induced by setting a rectangular cavity in the waveguide, a figure of merit as high as 6 555, which offer flexibility in the design of the structure. The waveguide structure may find widespread applications in highly integrated optical circuits, special for nano bio-sensor.