Due to the existence of diffraction limit, the optical performance of traditional photonic devices is limited greatly. Surface Plasmon Polaritons (SPP_S) can overcome the traditional optical diffraction limit and localize light in sub-wavelength range, so it has very important applications in the transmission, processing and control of optical waves in high-density photonic integrated circuits. For the advantages of low ohmic loss, long propagation distance and easy fabrication, MIM waveguide has become one of the most promising waveguides. Square resonators are often used in SPPs-based MIM waveguide systems because of simple structure and easy fabrication. Fano resonance has a sharp and asymmetric line shape and is very sensitive to the refractive index of surrounding environment. Compared with a single Fano system, multiple Fano resonances can realize multi-channel sensing and have the ability of parallel processing, which has attracted great attention. In this paper, a metal-insulator-metal waveguide structure consisting of a square ring resonator with a central rectangular air path and a bus waveguide with a baffle is proposed, which is studied by using COMSOL Multiphysics 5.4 based on the Finite Element Method (FEM). Because of its low power dissipation, silver is chosen as the metal material of MIM waveguide. The thickness of the structure is large enough (much larger than the optical wavelength), and the simulation results of 3D model and 2D model are basically the same, so the 2D model is adopted to reduce calculation quantity. The optical properties of the structure are studied, and the formation mechanism of Fano resonance is discussed according to the transmission spectrum and magnetic field distribution. In addition, the influence of changing structural parameters is discussed. The application of the proposed structure in sensing is summarized. Firstly, the transmission spectra of a single square cavity, a square ring cavity and the proposed structure are given. The square ring resonator with a central rectangular air path can increase the effective cavity length and change the propagation path of SPP_S in the resonator by providing more plasma resonance modes. Secondly, in order to understand the formation mechanism of Fano resonance of the proposed structure, the magnetic field distributions at the peak positions of Fano resonance are given. The simulation results show that the proposed structure can excite quadruple Fano resonance, and a filter band is formed in the transmission spectrum. The position and intensity of Fano resonance and the width of the filter band can be adjusted conveniently by changing the structure parameters. Thirdly, the number of Fano resonances can be adjusted by changing the side length of the square ring, and six Fano resonances and two filter bands can be obtained at most. At the same time, the filter width and the center wavelength can be regulated. The bandwidth of the proposed structure is defined as the wavelength range in which the transmittance is less than 1%, and the maximum bandwidth of the proposed structure is 275 nm. Therefore, the structure can be used for making band-stop filter. Finally, the application of MIM waveguide based on SPPs in refractive index sensor is studied. In biomedicine, it is very important to measure glucose concentration in the body, because it is often used to check the level of blood glucose. The proposed structure is very sensitive to the refractive index of the filled medium. The application of the proposed structure in the detection of glucose concentration is studied by investigating the relationship between the resonant wavelength of each Fano and glucose concentration. In order to evaluate the performance of glucose concentration sensor, the maximum sensitivity and Figure of Merit(FOM)of the proposed structure are calculated, which are 3 028 nm/RIU and 157.14 nm/RIU, respectively. The sensitivity and FOM value of the proposed structure are compared with that of MIM waveguide structure proposed in recent years.