Traditional optical devices have certain limitations, due to the diffraction limit, the development of optical devices is limited, making the optical devices bulky and not conducive to miniaturization. Optical devices can transmit and process information within the sub wavelength range, effectively addressing the issue. Metal-dielectric-metal waveguide can regulate the light propagation signal in the subwavelength range, and surface plasmon polariton has the characteristic of breaking the diffraction limit, making it possible to further miniaturize optical devices. Based on surface plasmon polaritons, a waveguide side coupled resonator system with metal baffles is proposed. When incident light enters the structure from the incident end of the waveguide, a narrow discrete band is formed in the resonant cavity, and a metal baffle generates a wider continuous state. When the narrower discrete state interferes with the wider continuous state, two different modes of Fano resonance lines are formed. In this paper, the finite difference time-domain method is used to simulate the transmission characteristics of the structure. The magnetic field distribution, electric field distribution, transmission characteristics, and sensing performance of the structure are studied separately. Based on the magnetic field distribution and electric field distribution, the formation mechanism of Fano resonance can be better explained. According to the analysis of simulation data, the geometric parameters of the structure and the refractive index of the medium can adjust the transmission performance of the structure. The geometric parameters of the structure are optimized. Firstly, when the side length of the Ag square is a=190 nm, the separation distance is k=10 nm, the coupling distance is g=15 nm, and the circular radius R of the structure is parameterized in steps of 10 nm from 270 nm to 330 nm, the quality factors at different R are calculated. Considering the overall quality factors of these two Fano resonances, the optimal value of the circular radius of the structure is taken as 300 nm. Secondly, when the circular radius R=300 nm, the separation distance k=10 nm, the coupling distance g=15 nm, and the Ag square side length a of and the structure is parameterized in steps of 15 nm from 160 nm to 220 nm, the quality factors at different a are calculated. In order to simultaneously consider the quality factors of these two Fano resonances, the optimal Ag square side length a of the structure is set at 190 nm. Thirdly, when the circular radius R=300 nm, the Ag square side length a=190 nm, the coupling distance g=15 nm, and the resolution distance k of the structure is parameterized and scanned in steps of 1 nm from 6 nm to 14 nm, the quality factors at different k are calculated, and the two Fano resonances are combined. Therefore, the optimal resolution distance k of the structure is chosen as 10 nm. Fourthly, when the circular radius R=300 nm, the Ag square side length a=190 nm, the separation distance k=10 nm, and the coupling distance g of the structure is parameterized in steps of 1 nm from 6 nm to 16 nm, the quality factors at different g are calculated. Overall, considering the quality factors of these two Fano resonances, the optimal coupling distance g of the structure is determined to be 10 nm. Under the optimal parameters, the figure of merit of the two Fano resonances FR1 and FR2 generated by this structure are 4.502×10⁵ and 1.967×10⁵, the corresponding sensitivities are 800 nm/RIU and 1 400 nm/RIU, respectively, the quality factor and sensitivity of the two Fano resonances have reached high values, both of which have good sensing performance. The designed coupling structure can provide a feasible way to improve the performance of micro/nano optical sensors.