Based on the resonance principle of waveguide grating and Goos-Hnchen displacement theory, a refractive index sensing model of surface coated waveguide grating is proposed, and the resonance spectral characteristics are analyzed. In addition, the resonance spectral characteristics are optimized by coating the polymer film upper the grating, the porous silicon is used as the sensing unit, the samples detected and electromagnetic wave will contact fully and the sensitivity can be improved effectively. According to the phase condition of the waveguide grating resonance, the relationship between the resonance wavelength and the refractive index of samples detected is established, and the concentrations of samples can be detected by detecting the shift of the resonance wavelength. The results show that the surface coated waveguide grating sensing structure has the sensing characteristics of high quality factor and high sensitivity, the quality factor of the surface coated sensing structure is 1 488, and the detection limit of refractive index is 5×10－4 RIU. Moreover, the sensing characteristics are analyzed by detecting different concentrations of glucose solution, the results show that there is a good liner relationship between resonance wavelength and the concentrations of the glucose solution, and the sensitivity of the glucose solution is 1.12 nm/1%, the validity of the sensing structure is proved. The surface coated waveguide grating sensing structure can be used to detect the samples with low concentrations and provide theoretical guidance for the further study of the optical refractive index sensors.