Based on the theory of Surface Plasmon Polaritons(SPPs) and the Metal-Insulator-Metal(MIM) waveguide structure, a waveguide structure was proposed, which consists of a square split-ring resonance cavity, a baffle and a MIM waveguide. The transmission characteristics of this structure is calculated by using the Finite-Element Method(FEM).The simulation results show a Fano resonance in the transmission spectra, and the resonant wavelength can be easily tuned by changing the length of the square split-ring resonance cavity or the width of the split. The sensitivity and the figure of merit of this structure is 1 600 nm/RIU and 1.3×105 separately. By changing the position of the split, double Fano resonances can be observed in the waveguide with a sensitivity of 1 700 nm/RIU and the figure of merit of 8.3×104 respectively. The waveguide structure may have wide applications in highly integrated optical circuits , especially for nano bio-sensor.